WillAdams wrote:some sort of clips or mechanical attachment to go in-between the Dual-MakerSlide on the X-axis (or the correct spacer(s) to use to eliminate the gap between the extrusions)
FWIW, I use M3 x 20 hex socket cap screws (with M3 washers) through holes in one piece of MakerSlide, into M3 insertion nuts (Misumi p/n HNKK5-3) in the slot of the opposite piece. I drill a 3 mm hole every 120-150 mm of rail, into the bottom of the single T-slot on the face with the Vs, through the core. The front rail has its slot in the lower position, the back rail in the upper position, so each grips the other one. The result is very stiff, and the holes are easy to drill and their position not critical at all. Also, it's not nearly as final as gluing: if you didn't get the two rails perfectly aligned from the first try, you can loosen the screws and try again.
Of course, this gives you no space at all between the rails, so no adjustment to make the rail fit the carriage. You have to make the carriage fit the rail instead: you have to figure out the spacer and washer combination that works. This is also true of any clip or other rigid connection between the rails.
A V-wheel assembly is, in theory, 11 mm thick, and the distance between the Vs of the back-to-back rails is, in theory, 40 mm, so you need exactly 29 mm of spacer and washer in between. I get there with one 25.4 mm spacer and some washers. M5 Form A washers are nominally 1 mm thick (actual thickness varies, usually around 0.95 mm) and Form B washers are nominally 0.8 mm thick (usually around 0.75 mm). It's not difficult to find a combination that adds up to the actual distance needed, but it requires some patience and experimentation (put two V-wheels with some washers and a spacer on a bolt, tighten firmly, and see how it rides on the rail; disassemble, replace/remove/add washers, repeat...). Once you find the winning combination (with my spacers and washers, that's three Form A washers and one Form B), you can use it on all four bolts, because the washers in one kit are likely to be identical.
WillAdams wrote:How many starts does the M8 threaded rod have?
AFAIK it's ordinary M8 thread, M8 x 1.25 mm. One start.
WillAdams wrote:Would it be too likely to result in one cross-threading the nut?
The carriage would be constrained near vertical by the MakerSlide, so I'd say cross-threading would be unlikely. However, that depends on how neatly cut and bevelled the end of the M8 threaded rod is.
WillAdams wrote:are the belt anchors supposed to interfere w/ the travel of the gantry and limit the work area?
It's obvious that they do, unless you trim them a bit. I'm not sure whether that counts as "supposed to", but they simply do. I made mine 12 mm wide, and even then, if they're a little off-center, they can touch and impede the V-wheel. I can get away with them being this narrow because they're 1.5 mm stainless steel, not fiberglass composite.
WillAdams wrote:would it be okay to put the top bolt for the Y-axis MakerSlide through the top-central round hole (above the belt anchor slot) thus gaining some additional Z-height?
I don't see why not, although the lower configuration is likely a little more rigid. As long as one is aware of the tradeoffs, the more ways to assemble it, the merrier.
