Of course. C rate is dependent on the battery capacity. 100Ah is 100Ah at any voltage, thus requiring 10-20A. If you have to put 50Ah in, you'll need at least 3 hours @ 20A.
If you put the 12V in parallel, now you have 200Ah. Now you charge at 20-40A. If you have to charge 100Ah @ 40A, you'll need at least 3 hours.
Lead-acid requires a fairly long absorption time to get truly full. The 3+ hours I mention will get you nearly fully charged and would be fine for occasional help, but the batteries should regularly be topped to to truly full, and that may require 2-4 hours in absorption phase alone.
LFP can typically charge 0.5C or higher with a very short absorption period - usually less than 30 minutes.