Not what you want to hear, I know, but...
I prefer the Windtech clamps for this kinda thing, too. The price of the Shure A27M bar gets you two SC's, but the same price also gets you one SC + two Windtech's with greater flexibility and stability for stand-clamping, as well as maintaining the option to clamp onto biggish stuff like railings, tables, chair arms, etc..
I find the SuperClamps are too bulky and heavy for most stand purposes (especially considering I run a compact Bogen 3373), and are better for biggish stuff. The Windtech's are nice and compact, so I'm able to easily carry 2-3 of them in the space a SuperClamp takes up. (Of course, I also carry a SC, but not for stand-clamping.)
The Shure bar resulted in my mics jutting out pretty far from the stand's vertical axis. (
Like this.) Perhaps this is what you mean by the "Shure bar is not suitable with the A51TL shockmounts"?
The SC, in its stock configuration, juts out even further and only allows clamping perpendicular to the stand's vertical axis, like the Shure bar. The addition of a right-angle mini-extension arm will resolve that problem. In fact, I recommend getting one of the mini-extension arms for flexibility clamping onto biggish stuff. But if you go with two SC's, you'd need two in order to prevent the shocks from sticking out so far, and it's just more stuff to schlep around.
The Windtech clamps, on the other hand, will work better, with a smaller footprint, for a lower price and allow you to run the mic's as close to the stand's vertical axis as possible, which helps with stability: