Some one mentioned a canted crossbar above.
If the cross bar is not straight then it's stressed to the point of being bent, which is really bad.
When assembling doubles you set the tanks on their backs and get the cross bar started evenly in each valve 4-5 threads and then slide the bands on under the tanks and get them properly positioned and spaced.. Then as you start to tighten the bands, you adjust the cross bar length as needed by turning it one way or the other to keep the tanks parallel. The cross bar should continue to be easy to rotate, even when the bands are fully tightened.
At this point the cross bar will also be straight, and the tops of the tanks will be level. If the bottoms don't match, it won't matter much if the bands are suitably strong - unless of course you drop the tanks. That's bad any way, but worse then the bottoms are not even as if they land square, the shorter tank will want to slip in the band and the direct in-line impact may be enough to do that and bend the manifold in the process. If the bands are slightly loose, that's guaranteed.
Other wise, if the tanks impact at an angle then the vector applied to the tanks in the band is reduced, and it take a larger wack to bend the manifold. But, if the difference in height is significant, it's not a bad idea to shim the short boot so they stand level, if you plant to use boots and stand your doubles upright.
I bought a set of doubles from a complete and total moron who shipped them with the manifold assembled and the bands loose in the belief they'd be less prone to damage that way. I can't imagine what he was thinking shipping them assembled instead of as two tanks and a manifold in 3 separate boxes. The manifold had about a 1/2" bend in it when it arrived and one of the valves was bent as well. The jerk never did pay for the damage.
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Steel 72s were made for decades by several manufacturers and getting a pair that match in terms of height, shoulder and neck profile and buoyancy profile can be a challenge, unless they are very similar age and from the same manufacturer and even that's not a guarantee.