There's no "Mixed 32/64-bit matching the platform" option equivalent in Merge Module project type because it's not needed. You can set their behavior by following the "Mixed 32/64-bit matching the platform" wizard in the main package.
For more information you can read the first part of the following article:http://www.advancedinstaller.com/user-g ... dules.html
FYI: A 32-bit installation package having an attached 32-bit merge module will install on both platforms.
All the best,