I want to build a mostly ground level deck. I have created plans and I want it to be 16' x 20' one long side will have a notch for a circle of rocks and a firepit. my issue is along straight the 20' side. I don't have 20' lumber available. How can I handle where two 10 foot boards meet? I've kept everything well below max span. I'll be using ground contact pressure treated lumber, and I checked it's southern pine.
You could double up on the Joist back to the blocking and just have one rim a bit longer than the other.
Not sure how you are fastening the Rim to the end of the Joists, but you could use a double 2x Hanger on the rim to attach the joist and its doubler from the inside with something like an HHUS26-2.
If you wanted to go Simple get a 10' and a 12' and then put the splice between the joists and then use the scrap from the 12' as a backing to bind the two rims together.
Another idea, I'd shorten that middle joist by 1.5" and double up the (looks like) 30.5" section of the rim joist between the adjacent joists with a piece 2x8 x 30.5".