I believe Detours patches the import table inside a single process, it does not patch the call on a system wide basis, so there really is no 'two different programs each patching the same function'. In theory you could have two different pieces of code running in the same process doing that (i.e. each patching a given function), but Detours gives you a 'trampoline' function to invoke the original thing you are patching so I believe the second to patch, when invoking the trampoline function would simply invoke the first patch, which when invoking its own trampoline would invoke the original, though I haven't tried that so it may not work that way :)

VS uses this mechanism allow you to run as non-admin even though there is TONS of VS and third party code that expects to do things like write to HKLM which is a no-no if you are not an admin.