From the 2008 FAQ
Quote:
Q. Can a Commander initiate a
combined assault with another
formation that has already performed
an action this turn or is broken? Does
participating in an assault with a
Commander prevent a formation from
performing an action later in the turn?
A. A formation may only participate in an
assault with a Commander if it would
otherwise be allowed to make an Engage
action, and doing so counts as their action
for the turn in all respects.
So only un-activated formations can be used in a combined assault. You only ever get to activate you own formations once.