There's little doubt that your safety margin increases enormously by using the dive partner system (I hate the term "buddy"). There's also little doubt that few spearfisherman employ its use on a regular basis. By its very nature, spearfishing is a solitary sport, where stealth, quiet and surprise all play in to a successful hunt.
Having a second diver hanging around in close proximity is counterintuitive to stalking. However, when properly employed, having an extra set of eyes, ears, limbs, and internal senses can actually make the hunt more successful, and prevent disasters large and small.
Before we go further, a word of caution. Having a partner watching your back can lead to a false sense of security, and a diver might take risks they otherwise might forego. Don't fall into this trap. Your partner doesn't want to go through the drama of a rescue, especially one that might be unsuccessful. Dive within yourself at all times.
I had a chance to make good use of the partner system while diving several islands in Mexico that were notorious for large predators. Having been hit by a large shark a month prior in the Gulf of Mexico, I wanted an extra pair of eyes to watch out for the large tigers that had been known to frequent the area. My partner for most of the dives was a friend who had recently steered off a Great White with a last minute shot that bounced harmlessly of its nose. We dove the islands for several days without incident, watching and signaling each other when a shark would show up.
I've also been helped by and have helped other divers retrieve guns, shafts, weight belts, and holed up fish. Having the second diver there provided a safety net in case of shallow water blackout (SWB), added a sounding board for ideas to search for and retrieve lost gear, and provided vigilance against stray boat traffic.
Finally, I've learned a great deal about diver movement and fish behavior watching other divers hunt. While there is no substitute for actually hunting yourself, watching others do so can teach you things you would otherwise miss when focusing on your own hunt.
What follows are some guidelines I've found important and helpful when diving with a partner. If you haven't tried it, you should.
It's inevitable that no two divers are evenly matched in skills and abilities. You and your partner need to be aware of each other's limitations. It makes no sense to attempt to extract a fish at 80 feet if your partner's working depth is 40 feet. Remember you'll lose anywhere from 20-50% of your bottom time because of adrenaline in the event of a rescue. Be honest about your conditioning, you may have been able to reach 60 feet at the end of last year's diving season, it's doubtful you'll have the same capacity after sitting on the couch all winter.
Train for the unthinkable
Until I attended the LeMaster/Krack Performance Freedivng Clinic I really had no idea how to rescue a diver in distress, it was a skill that hadn't even crossed my mind. Rescue classes are offered by any number of agencies and groups, some for little or no cost. Additionally, first aid classes will help you learn basic CPR and drowning assistance. A few hours one evening or weekend is a small amount of time to gain the basic skills to save a fellow hunter.
My primary dive buddy and I also annually practice diver retrieval, so the skill remains fresh and intuitive.
I had found that when diving with a partner, we often became separated. I would go one way, my partner the other, and within minutes we would be hundreds of yards apart. We resolved this by designating a "leader" for each dive. The "leader" would decide where we would hunt, moving from spot to spot. The second diver would follow the other diver as we moved. Combined with employing the "one up, one down" technique, we could stay together even in marginal visibility. Having to wait while the other diver hunted also help to keep a good surface interval for rest and recovery.
Go over hand signals, so that you can communicate clearly. I try to stick with the basic SCUBA signals whenever possible.
Finally, each diver should be able to "pull the plug" on a dive session. A tired, injured or otherwise compromised hunter puts both divers at risk.
Vigilance and Preparedness
The "up" diver should try to maintain a constant scan, in a 360-degree field, making visual contact with the "down" diver every 5-7 seconds. As the "down" diver begins to surface, the "up" diver should begin to breathe up, and should watch for signs of distress. If the "down" diver appears to be in distress, or has been down for an unusually long time, the "up" diver should time their dive to intercept the diver somewhere between 20-30 feet.
Before and after the shot
Guns should always be kept pointed as far as comfortably possible away from your partner. There are occasions where you may have an opportunity to shoot at the same fish, my standard agreement is to have the closest diver take the shot, provided it's well clear of the other diver. The non-shooter should try to back out of the shot zone whenever possible. If you've been nailing fish right and left, and your partner's coming up dry, try to give them a shot when you can (see "It's a partnership" below).
It's often tempting to fire at the fish that might be attracted by the fish your partner has just speared. Dorado and other pelagics are often curious and will school around the struggling fish, providing tempting targets. I determine whether or not to shoot on a case by case basis. If your partner is struggling with a particularly large or hot fish, if you are in heavy current, or if there is the possibility of predators arriving, your presence is better used focusing on protecting your partner from entanglement or attack. If you have any doubt, don't shoot.
It's a partnership
When diving with a partner, I share the catch equally, and try to provide equal opportunity for hunting success. Be open to suggestions and observations. By sharing information and hunting strategy, I find that success comes easier. Don't be a hog. It's often more rewarding to watch a diver get their first fish of a particular species or size, then to just add to your own stringer. Karma happens.
While the sum total of the ideas above might seem a bit militaristic to some, it really can be boiled down to a 5 minute conversation before each hunting session, and are habits that can be easily incorporated into your routine. It can also be the difference, literally, between life and death for you and your dive partner.