When I first moved to the Lot it was autumn and the river was still low after the usual long summer drought. I told myself I was back to freshwater fishing after a few decades mostly sea fishing, so I dug out the tackle I had used when small river fishing in my teens.
I had great fun with small barbel, chub and bream but lost a couple of big fish, the river was weedy and there were so many fallen trees!
As winter weather brought some rain the river quickly became too much for my initial tackle choices, I actually stopped fishing for months!
But during future winters I was frustrated enough to dig out heavier tackle, a couple of carp rods I had brought with me. The winter flow that I thought would need several ounces of lead I quickly found was not as fierce as it looked. A couple of ounces worked most of the time, sometimes three ounces if a bit of distance casting was required. When it got to the four ounce weights needed I usually packed up!
Weights of 2-3 ounces can be managed with any barbel rod so eventually I bought one, only 1.5 lb TC but it coped remarkably well and I was back to enjoying fishing with a rod that would bend. The carp rods were still handy for the long casts or swims where the carp had previously appeared and beat me up.
So I learnt not to be fixed in my approach to river fishing, and my results have steadily improved. Standard tackle choice for me now will be a 2.75 lb carp rod and either a barbel or feeder rod depending on the season or river state. I have short rods and long rods to cover any number of swims, some open and others very confined due to the trees.
If it's a new swim to me and I can fish two rods usually I put out one for carp, either short under the trees or long range. Either way I try to keep it clear of the main swim I fish for barbel, chub and bream. Many swims I know well now and usually will fish just the one rod, I like those swims better!
If I was to renew all my tackle I now have enough experience here to choose much better. I could probably manage with just for rods, rather than forty-four I probably have collected.
Choosing one or two for any particular days fishing would be so much easier!
For a reasonably clear (of trees) swim I usually take my Greys Prodigy Barbel rod 1.5lb tc and my Korum Big Water 13' 2.75lb tc rod. Both are 3-piece and pack down well for the camper-car. I have a selection of shorter rods for difficult swims, those rods are often butchered from longer models. I still "hate" carp rods but I guess they have some uses, if the river is running that hard I usually go home!
So, the original question was "one rod or two for barbel?" No straight answer, but usually take two rods as it gives options, fish one for the pleasure in a swim you know well.
Sorry, a bit too wordy to reach my answer, my excuse is that the forecast is to push the 40c temperature today and it's already too hot to go out anywhere!