By all means line up the transients and see if you like the effect...it can also sometimes work to offset them by just a small amount (not enough to be two distinct sounds, just a slight "stretch" on the thump).
However, as Track Rat says, phase doesn't come into this since it's two totally different sources. You can only match a phase relationship if you have exactly the same source miked from two different places (and you don't always want to do this anyway as it can be slight differences in phase that give you positioning information in the stereo field).
The pessimist sees the glass as half empty. The optimist sees it as half full. The realist just drains the darn thing and gets a refill!