I would doubt they original controller has enough capacity for a 260% increase.
I would be quite certain the panel's Voc is around 17 or 18 volts. Since you'll be replacing the controller anyway it should be easy enough to disconnect the wires from the panel and measure the voltage.
The two panels ideally would have identical Voc but being off a volt or less will make little real difference.
Keep in mind with your installation that you will need fuses in series with the panels. Really, only the 50 watt would need it but I'd do them both. Fuse the 50 watt panel slightly above it's Isc. If it ever shorts out this will prevent the larger panel from pushing nearly 3 times it's rated amperage thru the short.
2005 Chevy CC Dually D/A, 2003 Fleetwood 11X truck camper, 1997 Larson 206SEI boat