Doing some straight (theoretical) maths on your setup. Assuming 12V battery you have 1200Wh using the full rated capacity of the 100Ah battery.

2x50W will take 12 hours to fully charge that at full power. You'll never get 12 hours of full power to charge in a day I don't think.

Another battery isn't going to help you if you can't charge it. An extra battery would only help if you had excess charging capacity that the single battery couldn't use. An extra panel if anything would be what I'd be looking at.

Do you have an idea how much power you need? Your load should be fairly constant as it is now. From there work backwards as to how much energy/capacity you need. 1200Wh is a fair bit of energy. That should run a 50W load for 24 hours if fully charged. If the number of sun hours per day x 100W is less than your average power x 24 you'll go backwards slowly. So you're probably slowly going backwards with the shorter days.