What I do is just accumulate a list of packages which aren't
MAKE_JOBS_SAFE on my own and use some makefile rules to set
_MAKE_JOBS= if that package is being built.

If you *really* wanted to build multiple packages at once, you could
always use pkg_comp to set up two seperate chroot environments.  Just
duplicate the .conf file--setting the appropriate directories
differently, of course--and include the rest of your settings using
the EXTRAMK variable.

Regards,

--Blair