Try redirect one of the calls to the Bridge application. Note that bridge always returns to the dialplan, whereas Dial normally only does so for a failed call.
This may be pushing the limits of feature codes, as it undermines the application that was running when the feature code was issued, and possibly the channel on which it was running. As such you will have to try it, and plan to retest on major new releases.
We cant’t answer how to do this on FreePBX here, and paging groups are FreePBX constructs, not Asterisk ones.