Hi Claudia
Yep, you can fish for both out the same swim. Takes a little juggling. What I tend to do is take three rods - but never use all three at once. Fish a Barbel rod a third of the way out (or a quarter depending on where the ledge is.) Just over the ledge is best for the big ones - they seem to like a structure on one flank and this is specially true during the day. An example was this weekend, when I spoke to two ol' boys from somerset; they had been fishing all day with no success. Baiting up trent style. They were fishing well below a weir where the river narrows, but smack in the middle. One moved his rod to a closer and suggested line (you tend to see this line as there is a very slight eddy, and roach tend to hit surface.) Result = ten mins later a 11 14 barbel.
Depending on which swim you are in, from a rod length to a quarter way out, tends to be great for roach and dace/bleak. You feed a swim up - maggots - and fish for bait. This tends to get a shaol going quickly. Once you have collected some - bleak being the absolute tops for Zander - you plop in a tiny Zander float, with a lip hooked bleak. The other one, is if you have a deep hole next to a tree/cover. Then you can watch your float close in, and set the Barbel rod up self-hooking style. You can - and I often do - just keep trickling in maggots. This keeps a shoal there, if you needed more bait. It also brings in the Zander. You will know they are there cos as you concentrate the bait fish, they will jump out the water in a starfish shape, a Zander will often jump clear out the water as they attack them.
But all the above changes, with any decent amount of cloulered water going through.
The area we are thinking of fishing - and ****ie is sorting this with clubs etc at the mo'; has excellent Barbel (up to 17 - and yes this is above the claimed seven record!) and the Zander run at present to 16 and half - to a live gudgeon. But they are barely fished for in the section. On the day, if you meet me and ****ie early, we can put you in a good swim, for either B or Z, or both? I can show/give you a simple but extremly effective Z rig. (but you need a rod with largish rings at the end, and prob 12 pound line cos you may pick up a big pike) I'd also bring some wire cutters, long nosed forseps, and a pike glove - if you ain't use to handling things with teeth. I'd be very suprised - conditions willing - if you don't get a couple of each species over the weekend.
But, it might be best to spend a day barbeling, and a day Zangling (sorry Dic' knicked this term from you!)
Cheers
Jim