Multiple / simultaneous flows?

The product I'm currently working on comes with three different FPGA configurations, and these are each implemented as a "revision" in the Quartus II project file.

Often one would want to compile all three revisions together. I've written a TCL script that compiles each revision consecutively, basically like this:

#  Compile selected revisions.
foreach revision $revs_to_compile {
   set_current_revision $revision
   execute_flow -compile
}

However, this takes several hours and Quartus II doesn't seem to utilize my six-core CPU very efficiently, so I'd like to compile all the revisions in parallel to reduce the total compilation time. Is there a way to do this with a TCL script?

I could always run several scripts in parallel (i.e., several instances of quartus_sh), but I thought I'd ask if there's a better way. "help execute_flow" seems to indicate that there is, as one of the return strings is "ERROR: Only one flow option is allowed. Only one flow can be run for a single command call. If multiple flows are required, use multiple commands.", but I have no idea what this means.