Extending an outlet in the middle of a run
The main problem with this approach is that if you simply cut the cable in half, there won't be enough slack in the wires to get sufficient length-in-the-box (6" per NEC) for wiring the intermediate splice.  Oops!  I would extend from the garage outlet instead -- you can simply tap off its LOAD terminals and have GFCI protection for the outdoor outlet without putting another GFCI in.