The two are additive, but as always, one's absorption of transdermal T is unknown. However, androgel may disproportionately increase E2 so what you feel may be different from what you expect. Will also increase DHT, good for libido, bad for hair loss in those with genetics for that.
You cannot compare how you will feel with injections+transdermal VS injections alone even at the same T levels as FT, E2 and DHT will be different.
If thyroid is a problem, you may not absorb very well.