With a 12 volt battery system this MPPT can accommodate up to 200 Watts of panels. The 75/15 in the title refers to max PV voltage (75 V) and max output current (15 A)
If you do the math using the formula Watts = Volts x Amps you will see that they used the value 13.33 volts as the charge voltage to come up with that number. 13.3 Volts x 15 amps = 200 Watts.
So to answer your question, if you want to add 2 x 175 Watts panels you need another larger charge controller.