Hi There,
This was a tricky one actually, and I think this is a feature that might need some development.
For now though this should do what you need. Here you have 2 routines (one that contains slider 1 and one that contains slider 2) and a loop is used to randomly pick which one will be presented on each trial.
randomiseSlider.psyexp (18.5 KB)
conditions.xlsx (8.5 KB)
Hope this helps 
Becca