I think folks must be excellent at tap-dancing to do this. Today, as an example, I was trying to switch from a delay set to a harder tone set then back to delay and the hard part is really just understanding how to switch them at the right time otherwise you get such a horrible noise (even with a bass). But its getting better and this is like anything else with music, practice practice practice.
I used to think that using a looper was hard and then this has now come up. The good news is that I am not alone as the band guitarist also suffers from this particular affliction.