I'm looking for an idiom (if there is one) which captures the following meaning: trying to do two things with the same tool will never work, since every task requires its own tool.
It is similar to "if the only tool you have is a hammer, to treat everything as if it were a nail", but this has the emphasis of over-reliance on one tool. Rather, I'm looking for something that expresses the futility of trying to find a single solution for two different problems.
There's a saying that goes, If you run after two hares, you will catch neither.
(Prov.) You cannot do two things successfully at the same time.
[The Free Dictionary]
There's also this saying - "have your cake and eat it too"
to do or get two good things at the same time
[The Free Dictionary]
Usage:
"Trying to solve two different problems with the same tactic is like having your cake and eating it too. You simply can't."
Or you could simply say that it is foolish to expect a panacea every time you face problems. It's not an idiom per se but conveys what you intend well.
something that will solve all problems
[Cambridge Dictionaries Online]
