First off, you'll have plenty of time. RDCT starts at 9:25pm and Fantasmic starts at 10:30.
In order to get the best possible seat for Fantasmic, watch the fireworks from in the hub over by Frontierland. When the fireworks end, you can follow the masses heading over to the Rivers of America to get seats for Fantasmic. Be aware that this won't be very "quick" because the crowds in the hub are packed in like sardines, but you should be able to get a decent enough seat.
Please note that if your RDCT seats are near Adventureland, you're going to have to go through Frontierland to get to the Rivers of American anyways. Crowd control CMs will repeatedly tell you to go around.
I watched Fantasmic after the fireworks in June. I waited about 15 minutes after the fireworks ended to let the crowds thin out some and joined the masses heading to Fantasmic. I eventually found a spot I liked since I didn't feel like walking anymore, but it was slightly obstructed by a lamppost.
If you have younger children, you may want to consider splitting the shows up to get better Fantasmic seats. I would do RDCT first because the fireworks show can and does get cancelled, but Fantasmic is far less likely to be cancelled since the pyro is minimal.