Well I would guess that there are more 64-bit Macs than 32-bit Macs out there, but anyways, ideally an application should be distributed as a fat binary containing both architechtures (which isn't that hard to do).
I tried recompiling nme for 64-bit, but it wouldn't work because sdl-static is not built for 64-bit on the Mac. I don't see 64-bit Mac support anywhere on the sdl-static page, so this the end of the road...?
When I try to a haXe nme game in 64-bit mode like this:
haxelib run nme test "Actuate Example.nmml" mac -64
I get this error:
Called from ? line 1
Called from InstallTool.hx line 384
Called from a C function
Called from InstallTool.hx line 70
Called from a C function
Called from installers/InstallerBase.hx line 95
Called from installers/DesktopInstaller.hx line 180
Called from installers/InstallerBase.hx line 172
Called from installers/InstallerBase.hx line 367
Uncaught exception - Error: Source path "/usr/lib/haxe/lib/nme/3,0,1//ndll/Mac64/nme.ndll" does not exist