Oh good lord, just answer the man's question.
The center to center spacing on either 7.25" or 8" tanks is usually around 210mm to 220 mm. Most isolator manifolds will fit most tanks in suitable bands.
They come in basically two flavors - face seat and barrel o-ring varieties. OMS sells a face seat manifold and it has two large hex bolt looking connections on each side of the isolator - similar to the old cone seat manifolds used back in the day. They are not adjustable as the center to center distance is what it is.
The barrel o-ring manifolds have a thinner looking crossbar that screws into the left and right valves and the seal comes from two o-rings that seal inside a barrel that allows a lot more range of adjustment to accommodate different bands. Most people go with that design and several companies sell them.
Diving doubles is not magic nor is it a real problem to use them on recreational dives. I normally take double 100's on boat dives and just use half on the first dive and half on the second and don't have to bother changing tanks.
The only cautions are that you do have a lot of gas that can get you into serious deco, and you really need a BP wing to provide good stability with them. The swing weight is also twice as much as rather than 5 lbs of gas on baord, you have 10 or 12, so you will get a lot more buoyancy shift from full to empty.