I was figuring to use them as a 2 tank dive. My understanding, and correct me if i am wrong, the advantages of diving both tanks its that its easier to keep in trim with the 2 tanks.
- It really depends on the tanks used. A single AL80 (the ubiquitous tank in most vacation destinations) only shifts from marginally negative to marginally positive. It shouldn't cause any destabilisation.
That said, it really depends on how you've configured your rig and attached the tank. If it's snug against the torso and not dangling or swaying it's fine. But if you've configured badly and the the tank has a pendulum effect, or isn't close your lateral centre-of-gravity then the disbalance will be exaggerated and it'll feel very unbalancing.
The key is proper instruction and optimal rig set-up.
Also say you are planning on turning the dive at 1000 psi and being back at 500 psi this means you can turn at 1875 per tank and return at 1625 for the first dive and 500 / 225 essentially getting an extra couple hundred psi per dive while maintaining the reserve.
If you have a modicum of gas planning and situational awareness it's pretty easy to use two cylinders across two dives.
Just set yourself a 'rock bottom' for dive #1 that preserves sufficient gas for dive #2.
Personally, I see little validity for single tank sidemount diving. Not because of balance issues, but because there's no real logic in losing the safety of gas redundancy.
If you dive with one tank, how much does it throw you off trim? I would assume a pair of pockets with a couple of 2lb weights would work well for this and just swapping pockets as the tank empties.
It shouldn't throw off your trim, providing your configuration is optimal. Weight pockets are entirely unnecessary.