A tubescreamer is compressed enough as is...in fact, IMO, they are too compressed but in answer to your question, Stevie used one for grit/crunch and the other for a boost with very little gain...just a volume boost
I use to run a TS-5 with my TS-9 into my old Twin. The TS-5 was set for gain The TS-9 for boost. With both on I got a real thick tone. SRV used a TS-808 for boost only according to Warren Haynes. .
If you want compression and Tubescreamer-like grind, check out the Visual Sound Route 66. It's a pretty cool pedal for anything from blues to hard rock.
I use to run a TS-5 with my TS-9 into my old Twin. The TS-5 was set for gain The TS-9 for boost. With both on I got a real thick tone. SRV used a TS-808 for boost only according to Warren Haynes. .