The conditions to use free air ampacity table are kind of gnarly and I've given up on using them. Outside of overhead wires I think it's really hard. For instance you could argue that the solar panels are obstructing free air flow ?
You need to add both current since they will both be producing at the same time. In a fault I believe you assume that the faulting panel does not contribute current, which is a different situation.
So the 300W panel here has unusually high current for the size, somebody designed this to have low voltage / high current (and you probably want to do some voltage drop calculation to see how much you're leaving on the table with this current / voltage combination. It might be pretty high). So you would go over #10 limit. Depending on if you believe that solar implicitly has a 35A exemption to the 30A rule (which I think it does). 35A is way over 30A if you believe in the 30A rule, so I would step this up to 50A.
Check my
posts on this thread on a splitter that is 30A in / 50A out. If 2P you can do this just with MC4 branch connectors. After the combining you need to use the special MC4 connector along with #8 wire. However, since you need fusing on the smaller one, there is more reason to use a combiner since you need a place to put the fuse (you need to decide whether you prefer inline MC4 fuses or combiner box).
(Not sure if mentioned already but the list price for BougeRV panels is often pretty high. I guess the 300W is sort of a specialty panel, but one could also go for standard 300W panels (which are often super discounted since they're old) and adjust system design. If you have MPPT then the voltage is not a big deal.)