I backmount a HP100 and sidemount an AL 80 frequently. 1+ for the posts above. Make sure to ballast for the gas you use. The thing I like about it besides a good source of redundant gas is that I can use more of the fill than diving a single tank. The other day I dove for 100 minutes using both tanks. The next dive I kept the HP100 with 1900 left and sidemounted a fresh AL80. I would have never started a dive with just the 1900 left in the HP100. Used most of that up with enough left in the AL80 for its use the next day.
To keep my redundancy options fully open I swap which tank I'm breathing from every 500 psi. That is not for trim, but to keep some gas in each tank.
Most of my diving is shore based. 2 sidemounts take more effort for me than having one on my back and clipping the other on after getting in the water.