Had those many years back. No problem with coherency at all, if set up right. Two stereo amps, if of the same make, are better than one. Four monos are even better.
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My personal experience with stacked quads, driven by Quicksilver 8417's, was that I vastly preferred singles with the panels mounted on open stands that lifted them about about 15" off the floor with the faces more vertical. The sound stage was much larger, more bottom end, and better focus.
I have also heard the Quads with the Sequerra Ribbons driven by Electrocompaniet 25 w class A power amp, the ribbons do improve the image specificity.
Another alternative I've heard was to remove the 3 panels from the original frame and stack them in a base/treble/base vertical array. This again opened the soundstage up compared to standard, but the best result I had was the 1st paragraph above.