I like both names, and although they do sound a bit similar, I think they go together okay. Yes, they could both be shortened to "Lew", but what are the chances of that? And would it matter so much if they did? My little brothers have completely different names, but both have/had the nickname "Monkey". That sometimes got confusing, so sometimes it doesn't matter what the original name is.

I have a friend who has a Liam and and Liana, and as far as I know, very few people notice the similarities in the two names.