If you have a 10 amp controller, it will get maxed out by the 100 watt panel, so you could not install a second 100 watt panel. Yet if you get a new 100 - 150 watt panel, and install another 10 - 15 amp controller, no worries, both cab be connecte to the same battery, and each will charge at it's maximum capaicty, until it reached the controller max battery set point, then both will either reduce ampeage to the battery or shut off until the battery voltage is lower.
SunElec.comSolarOnSale.comFred.