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.
That is called the rim joist and in this case does not take load but just ties everything together, and very little of that as the deck boards will do most of that work. It's very common and very typical to just splice in the middle of a joist with 3/4" of each piece on the 1-1/2" wide joist and just nail each just like the other joists (number and size of nails), but a slight angle so the nails are fully in the joist