Which Groupset?

If you...allow the rear mech battery to run out, you can swap the front mech battery on to the rear, foregoing front shifting for the journey home.

Good move - I love Campagnolo but there's no way they would do that.

You upshift at the rear by pushing a paddle behind the right brake lever. You downshift at the rear by pushing a paddle behind the left brake lever. You move the front mech by pushing both paddles at the same time.