so you can't use backmount plates and wings to do sidemount, it doesn't work. You can use SOME sidemount rigs to do backmount, but the only one that does it halfway decently is the Dive Rite Nomad XT. It is far from necessary however. The wings for single tank and twinsets/doubles are not compatible from one end to the other. That is the issue, you essentially have to commit to either backmount or sidemount at this point. If you choose sidemount, you have to decide if you are diving steel or aluminum cylinders, as the rig configuration is largely different, it is more of a "style" difference, and both rigs are capable of doing both types of cylinders. If you choose backmount, you have to choose singles or doubles, and if you choose both, you will have to purchase two separate wings, one for singles, one for doubles. The backplate and harness is universal, though with doubles you typically do not need a heavy steel plate, so most people have a plate for singles and one for doubles