Fri Aug 21, 2015 7:16 pm by MiRai
You didn't mention the game you're playing, and while some games don't like several Actions sent at the same time, it should be possible. You pretty much have two choices:
1) Add a Do Mapped Key Action to the same Step of this Mapped Key in question, and then point it at your already existing assist Mapped Key in (assuming) Non-Combat with a Target of Window:Current.
2) Add a second Step to this Mapped Key in question, move the existing Step 1 to be Step 2 using Alt+Arrow Keys, and then, using the same instructions above, add in the assist Mapped Key via a Do Mapped Key Action to the, now blank, Step 1. Then set this Mapped Key to execute when pressed OR released in the main (top-level) settings.