I assume you want the formula to either read:
='Sheet1'!A1&"/"&'Sheet1'!A2
(so: textual concatenation of A1 & A2 with a "/" in between)
or
='Sheet1'!A1/'Sheet1'!A2
(so: the result of dividing A1 by A2)
Check the result of your formula: it generates neither :) (it results in an invalid formula) And, as Jordan answered, your VBA used a colon instead of a double quote.
UPDATE (read over the fact that you asked for textual concatenation in your question, and also adding a solution)
It's best to check the result of your formula build-up while stepping through VBA with the debugger (F8): it generates an invalid formula, on which Excel will no doubt give you an error when you try to set it. The result doesn't contain any &
characters to concatenate the values of A1 & A2 with the "/" in between.
The correct formula should be:
='Sheet1'!A1&"/"&'Sheet1'!A2
This is achieved by:
ActiveSheet.Range("A1").Formula = "='Sheet1'!A1&""/""&'Sheet1'!A2"
(note that you can embed a " in a VBA string by doubling it; in this case that is clearer to read than using Chr(34) for the "'s)