Presume your inverter has 2 x MPPTs, East side into one and West into the other?
You're trying to stay within the overall dc input watts from PV?
You'll have a max volts dc input spec for each mppt though, if you're at the max inverter limit with 8:8 then sounds like you would be exceeding the mppt input V limit by increasing the string from 8 to 10 panels? Not to mention voltage increase as temperature drops. Any opportunity to switch in/out panels would be limited by that.
Sorry I don't see how it could work in that case
Not to mention how the cables from the panels physically feed into the inverter.
If a bigger inverter is not an option then a second small pv inverter for the spare panels could be worth a look but the startup voltage could be a problem with only two panels
You might find the inverter has a high enough input pv dc current limit to re-wire the panels in parallel, 2x5 on each side of the roof, that would bring the string voltage down but increase the current. I think that's unlikely to work though, but check the numbers. You'll probably exceed the input pv power limit that way too.
Just my 2d worth, I have no practical experience on this...
