How about some more information? I.E. How big is your train table? How many loops of track? What size wiring?
You say you have two power bricks? What Type? If they are Lionel 180w bricks. One brick will power a decent sized layout. You may not need two power districts.
I have 1 Lionel brick powering my whole 10'x16' layout without issue. a second brick is dedicated to accessories.
The 993 is just an additional Legacy remote with a charging station as a base. The 993 base does not send commands. Only 1 base is needed per layout and you have that with your 990.
The 993 handheld merely needs to be set to the same channel as your 990 base as well as have the same software and it will automatically display the engines you have loaded into the 990.
Engine information is actually stored in the 990 base, not in the handheld.