I think the key is making sure you are getting good gas exchange, even when working hard. If that becomes difficult, then you are working too hard and need to stop.
It's easy to become stressed trying to keep up and then start breathing shallow, which only makes everything worse. The reg can give you all the gas you need, but if you are "panting" instead of breathing, you aren't getting it. You are going to accumulate CO2 because much of what you are breathing back in is CO2-rich gas that is in the "dead space" of your windpipe - the last bit of your exhalation that didn't make it out of your mouth. You need to exhale deeply and inhale deeply, or you will continue to build CO2 and feel even more out of breath.
Your OP says you starting getting water in your mouth. Maybe that was just because you were panting instead of breathing and the exhaust valve wasn't keeping up with your aggressive and rapid in/out. But, I would check that for sure. Do a check before your next dive. With the regulator on a tank and the valve closed, you should not be able to inhale. exhale and then inhale a bit several times. That flapper valve should lock shut every time you try to inhale and you should get no air at all.
A pony bottle is nice to have if you run out of back gas, but it won't solve a breathing problem. At best, it would help after you've gotten your breathing under control and you find you are now short of gas. In fact, when you are hyperventilating it is very, very difficult to switch regulators, sometimes impossible. The solution is to stop, completely, and work on breathing deeply and getting a good gas exchange. Shallow up because everything is easier shallow and your gas, whether on your back or in the pony, will last longer.
In your case, if I were at 60' was struggling for breath and couldn't get my buddy's attention, i would:
1. Stop.
2. Focus on my breathing, trying to get deep relaxed breaths and really get good lungfuls going back and forth.
3. Ascend slowly and under control.
4. If I got my breathing in order, had the gas, and could still see my team, I'd follow at 30' looking down on them, because my gas will last 1/3 longer at 30' than 60' and I'm that much closer to the surface if I have the same problem again.
5. If I've lost the team, I'd follow whatever protocol you've agreed on for buddy separation. Usually that's to surface after 1min. You could probably follow their bubbles on the surface.
It's easy to become stressed trying to keep up and then start breathing shallow, which only makes everything worse. The reg can give you all the gas you need, but if you are "panting" instead of breathing, you aren't getting it. You are going to accumulate CO2 because much of what you are breathing back in is CO2-rich gas that is in the "dead space" of your windpipe - the last bit of your exhalation that didn't make it out of your mouth. You need to exhale deeply and inhale deeply, or you will continue to build CO2 and feel even more out of breath.
Your OP says you starting getting water in your mouth. Maybe that was just because you were panting instead of breathing and the exhaust valve wasn't keeping up with your aggressive and rapid in/out. But, I would check that for sure. Do a check before your next dive. With the regulator on a tank and the valve closed, you should not be able to inhale. exhale and then inhale a bit several times. That flapper valve should lock shut every time you try to inhale and you should get no air at all.
A pony bottle is nice to have if you run out of back gas, but it won't solve a breathing problem. At best, it would help after you've gotten your breathing under control and you find you are now short of gas. In fact, when you are hyperventilating it is very, very difficult to switch regulators, sometimes impossible. The solution is to stop, completely, and work on breathing deeply and getting a good gas exchange. Shallow up because everything is easier shallow and your gas, whether on your back or in the pony, will last longer.
In your case, if I were at 60' was struggling for breath and couldn't get my buddy's attention, i would:
1. Stop.
2. Focus on my breathing, trying to get deep relaxed breaths and really get good lungfuls going back and forth.
3. Ascend slowly and under control.
4. If I got my breathing in order, had the gas, and could still see my team, I'd follow at 30' looking down on them, because my gas will last 1/3 longer at 30' than 60' and I'm that much closer to the surface if I have the same problem again.
5. If I've lost the team, I'd follow whatever protocol you've agreed on for buddy separation. Usually that's to surface after 1min. You could probably follow their bubbles on the surface.