What is the maximum span for a beam made of two 2x10s screwed together?
The Code requires a Live Load of 40 lbs. per square foot (psf) plus a Dead Load of 10 psf for a total load of 50 psf.
You indicate the beams are 8’ on center, so the total load on each beam is:
50 psf x 8’ (4’ on ea.side of beam) = 400 plf
In addition, the code requires an adjustment for wood being pressure treated. If wood is pressure treated without incising then it’s a 5% reduction. If it is incised, then it’s a 20% reduction.
Summary:
Depending on the species and grade, 2-2x10’s can span 12’ with pressure treated lumber without incising and 10’-6” with incising.  (I’m using a species of SPF and a Grade of No.1)
Note:
If the beam is along the edge of a deck and is loaded with a single span, then 2-2x10’s can span about 15’ without incising and 11’ with incising.