I use a 3l steel pony for dives down to about 25m (80ft), any deeper and it's twinset territory. When I carry a pony I have it mounted on my main cylinder, on my right hand side. Main tank has a primary and an octo, with the octo clipped off on my left shoulder D-ring. The pony has one second stage which is necklaced.
The pony is my emergency bailout, so I don't need to be able to hand it off to anyone. A gauge is an extra failure point, and I check the pressure before every dive using my main regs. Necklacing puts the reg where I can easily reach it. If I have to use it, then the dive is over and the only palce I'm going is up.
If my buddy needs air, then they use my octo as they've been trained. This gives them access to the contents of my main cylinder, which will always contain more air than the pony. The dive is still over and we're going up.
I don't change my weighting, and have never felt myself to be unbalanced.
The pony is my emergency bailout, so I don't need to be able to hand it off to anyone. A gauge is an extra failure point, and I check the pressure before every dive using my main regs. Necklacing puts the reg where I can easily reach it. If I have to use it, then the dive is over and the only palce I'm going is up.
If my buddy needs air, then they use my octo as they've been trained. This gives them access to the contents of my main cylinder, which will always contain more air than the pony. The dive is still over and we're going up.
I don't change my weighting, and have never felt myself to be unbalanced.