you don't mention what barrel length your xd40 is (i'm assuming the 4" ish size).
for what you're talking about, it would take a real loose holster. you've got differences in slide width and heigth, different profiles with your trigger guards (1911's sa vrs. xd's dao type), and of course barrel length.
uncle mike's has some IWB holsters that i HATE, but they might serve your need if you're dead set at only one holster. i'd get the one that would fit a 1911, and your xd might fit ok, might just fall down into the holster until your frontstrap comes into contact w/ the holster and that would'nt give you a good weapon presentation (when you first grab your pistol to effect a draw).
i STRONGLY reccommend that now that you've got 2 good firearms, if at all possible, get a good holster for each. i know that finances can be tough sometimes, but comfort, weapon security, concealability are too important for a one size fits all. that's why they are hard to find, no one has a really good one IMHO. don't skimp if at all possible.
if i were you, i'd get one holster for one of your handguns, don't forget the belt (get a good one- i use a 1 1/2" horse hide double thick from ted blocker), and just carry one for the time being.