I'd just join them together - it's going to be a nuisance placing the two objects together like that every time you want to place a wall. Udk counts each section as a drawcall. A section is split by material ID then split again at the object level. So your wall is going to be two sections no matter what you do.
Yep just combine the 2 together. It would be awesome to have them be all the same material, but if you are just going to place them together then its just better to have them combined. Unreal has draw call limits and asset limits. The more objects that it needs to cull out can end up being expensive even if they aren't…