Yes, the 64 bit Solaris implementation for AMD64/EM64T is closely modeled
after the 64 bit SPARC support; there's only one release to install and
that release supports both 32 bit and 64 bit binaries simultaneously,
when the default 64 bit kernel is booted.
As the number of general purpose registers was doubled, 32 bit applications
enjoy better performance on 64 bit kernels as the kernel runs faster.
They can also address the full 4GB of memory as the kernel no longer
need to carve out a bit for itself.