The 50 watt panels are 2.7 amps each for 5.4amps adding another panel will bring it to 8.1 amps 1.9 amps under the 10amps of the controller. BTW midday yesterday I was getting 5.5amps leaving the controller and 5.4 at the batteries.
In reading Renogy suitcase owners manual, it says charge controller Max PV input power-120 watts (12V). So I guess the answer is no on the 10amp controller. It's not much more to get one with another 10 amp controller. Obviously not like the one I have.