Change the platform option to be X86 and only reference the 32-bit dll. This way, the executable will run both on 32-bit and 64-bit OS. Application built with x86 option will run as 32-bit process on 64-bit OS under WOW64.
Also the next link can help - Conditionally use 32/64 bit reference when building in Visual Studio
The next link show how to load the right dll which is what you are looking for