I am not familar what exactly to do to compile 64Bit app/plugin on MAC. From my understanding a univrsal binary (Intel 32/64) does already include a 64Bit version, right? Do I need to compile a 64Bit version then in addition or is a universal binar 32/64 enough?
(I stick a check in my build script to make sure I've actually got a universal binary each time. The number of times I've thought I had one, but had accidentially changed a setting somewhere ...)
If You make some plugins. You can easly check if You perform correct setting (universal binary 32/64) when You run 32 and than 64 bit host. I always check differences in Reaper 32 and 64. If Your plugin have only one architecture than You'll see it in plugin list.
For example:
I compile my plugin as 32 bit bundle.
When I open 32 reaper I will see plugin name on the list : "myplugin.vst"
When I open 64 reaper I will see : "myplugin.vst (x86)"