The ratios aren't exactly 1:1 and 2:1. They're just convenient integers close enough to differentiate between them.
I just measured a Shimano RD - it moved 30mm with ~17.6 mm of cable, very close to 1.7:1. That's based on shifting high to low on a Shimano 7 speed HG cassette, which Sheldon Brown says has 5 mm spacing (6 steps x 5 mm = 30 mm) The ratio I measured fits with the 1.7:1 figure
given here for Shimano. According to
this table, SRAM is actually 1.1:1
So, I wouldn't expect it to work at all.