I don't own Halcyon gear myself. But, looking at the photos online, yeah the Eclipse and Evolve look to be standard backplate, standard wing, and standard harness. Which are 3 separate, modular pieces of equipment: part of the beauty of the BP/W system.
I use one 32-lb wing for single tank diving. I recently acquired a 44-lb wing for doubles. I intend to use the same exact harness and the same exact backplate for both. It's quite easy to swap out the wing, and many people disassemble the tanks from the wing and backplate anyway when they go in for a fill. So yes, I think you could buy one of these setups, and swap the wing out later to facilitate going from singles to doubles or vice versa.
By the way, some folks will say that you can dive a single tank with a doubles wing. Which is true, but it sucks. You're much better off with separate wings. There does not exist a wing which is appropriate for both singles and doubles diving, no matter what the marketing people tell you. I don't think Halcyon tries to market a product like this, but many of their competitors do.