That power supply (20 amps) is "almost" enough to run two controllers with the load you are describing.
I have about 20 feet total of 5050 Flex strips running from a 4.5 amp supply on a single MR16. Because I was concerned that it was marginal, and I didn't have an ammeter to check the total amp draw for all on, I did a check of how much the voltage dropped.
The input voltage to the MR16 is 12.2vdc at no load and drops to 11.9vdc at full load.
I made the command decision that it could handle the load.
It has continued to do so and is intensely bright.
On another MR16, the load was 200 RGB Pixels. The same small power supply choked big time. It didn't hurt anything, but the voltage dropped so low that the PIC chip reset until the voltage recovered.
So, to answer your question more directly, a 20 amp power supply for each or your described load is overkill. If you found a 25 amp supply for not much more, you wouldn't need as many, therefore cheaper. That same ebay company has 10 amp and 15 amp power supplies for less if you want to power each one separately.