shib - the combination of tennis and squash played in an integrated way does not mix for some players, including me.  And, not that that really matters unless you are trying to play representative level.  The stroking and timing are different so that means it takes a while to settle into the respective games.
Squash is good for tennis players because of the quick reflex and fitness aspect, as well as movement eg Mal Anderson (a top National tennis player of the 60's) played top pennant. 
Tennis is good for volleying in squash.
The least effect, I think, would be to play both sports in the off-season, if winning is of primary concern.

By the way, a true story -  one year we that this gun junior down to the city for the State U/19 Championships, and staying at the motel was boring, so during the day he went with his parents and played tennis.  This guy was top favourite for the title, and he bombed out badly that night in his match.  That was a bad choice in my books.

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It shouldn't have a negative effect. You may find your volleys to be improved by squash as well as your movement. Any improvement in racket head control helps for both sports.

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