I'm in favor of a clear separation between the forums. They are made for different audiences and not everything that is meant for one is meant for the other. As somebody who writes some pieces that are meant for both audiences, the cross posting feature is somewhat convenient for me (but not hugely so; I can just copy and paste). And as a reader, sometimes it's nice to see a post is cross posted so that I can go see the comments on the other forum.
I'd be interested to see how much the easy cross posting has increased the number of cross posts, and if so what kinds of posts are now more likely to be cross posted. This seems like an analysis the forum team could do and is harder to do anecdotally.
The EA Forum and LessWrong have some of the best technical infrastructure on the internet, and I think the EA Forum derives huge benefit from that. However, it does make me a little uneasy that it's made by the Lightcone team, who are in charge of LessWrong. I like the people on that team, but I expect probably some decisions that are good for LessWrong but not so good for the EA Forum might just end up propagating here by default. This is just a suspicion; I don't have any particular examples.
A lot of separation does exist. LessWrong posts are moderated pretty differently, the commenter audiences are often very different, and the types of posts are mostly different. So the connection as is isn't currently a huge concern of mine.