pardon the ignorance, but can this device be used for bar end shifters and sti shifters at the same time? If not is there a product that can be used for both and if not, why?
No, and yes, kinda…
Theoretically, maybe it would work. Would you have to mount 2 of those things on your bike, one for each set of shifters? That would look awful and add unnecessary weight. I’d suggest the traditional road bike set up with jammers on your drops.