The correct dll will be loaded by the COM system, not your code.
As long as both versions are correctly installed and registered in the registry, your code will work without any modifications. Note however that you will not be able to install the 64 bit version of the dll on a 32 bit machine.
You might want to look into using RedemptionLoader - you will skip COM registration completely, and RedemptionLoader will load the right dll for you based on the process bitness.