A single MSI cannot both install files or registry to 64-bit locations on a 64-bit system and run on a 32-bit system. So assuming your 64-bit ActiveX control must be installed to or registered in a 64-bit location, you cannot do this in a single MSI.
If you have the premier edition of InstallShield 2012, this is one of the use cases of the Suite (or Suite/Advanced UI) project type. You must create two MSI files, and include them both in the suite with relevant eligibility conditions. (The exact conditions depend on whether you make the MSIs standalone or cumulative, but the 64-bit MSI should only be eligible if the platform has an architecture of x64.)