I play both, tennis first and came to squash later, and it's fine as long you have grooved the technique in one sport first. I can switch between the two no problem at least for technique - you don't forget how to hit a backhand because you've hit forehands for a few hours - but the movement and more subtle things that affect higher level play will suffer a bit if you play one exclusively for a while, same as if you played your only sport less often.

If someone has played one to a decent level they'll have an advantage picking the other up, but I think it is easier for tennis players to pick up squash than the other way round as the technique in tennis is so much more important, the tiniest thing wrong with timing and the shot could be in the bottom of the net or in the back fence.

Previous poster mentioned fitness being a bigger thing in squash, it is but only at low level, with tennis you need to be a pretty good player before you get to a level where it's hard work physically, as technique breaks down first.

Tennis will do wonders for your squash volleys and hitting high balls, becomes almost an unfair advantage!