I use the drennan isotopes with their clips. One right on the tip and the other set back 8inches or so. Im normally fishing for Chub and Roach with the rod down. I would think you could get away with one isotope for Barbel.
Only trouble with fishing into dusk is that dusk quickly becomes dark and with no natural light even white tips are hard to see, bite alarms that show red light led's are much easier on the eyes than isotopes or starlites or torch's on a rod tip, goes without saying that baitrunners are the order of the day (or night in this case)