You'll want to set the zoom based on the distance between the two. Try:
Set zoom to dist(player1.x, player1.y, player2.x, player2.y)
You're going to have to tweak the equation to get it to zoom the right amount, like multiplying it by some amount, but that's the expression you'll use.
I can't remember, is zoom 50 smaller or larger than 100%? The value might be backwards.