FYI, just use one. Seriously, I've seen people try. It seems like its not long enough but don't be tempted to out think it.
1) Drill it square and centered,
2) lube with penetrant or hopefully tapping fluid,
3) put in the tap carefully centered,
4) get it started SQUARE to the hole,
5) if it starts to bind up, back off 3/4" of a turn to clear the chips before going further,
6) clean and degrease well,
7) a couple big drops of red (241) Loctite on the outside of the heli-coil,
8) install with the 'tool' until the end is almost flush with the inside,
9) break off the tip thing
10) Don't mess with it until its dry!
11) re-install according to the steps in the service manual that of course you printed out and have with you in the garage.
12) Enjoy beer while Instagramming pics of your manly repair