Yep, what greycat said. I assumed you wanted multiple channeled tiles next to each other for multiple wells.
That would be potentially dangerous if you have multiple miners, and if you designate them all at once. Again, one miner
might will stand on a tile that another miner is channeling, and then Fun Ensues.
If you want to designate adjacent channels over an existing water/magma source without anyone falling in, you either need to restrict the work to a single miner (by burrowing or physically constraining all the others), or designate one channel at a time. (Or a subset of the channels at a time, such that no two adjacent channels are ever designated simultaneously. This can get tricky.)
If the reservoir hasn't been dug/filled yet, you can designate the channels first, and let the miners have their way. They'll spend most of their time digging on the lower level, under the channels. Once that's done, remove the ramps at the edges, and dig the rest of the reservoir area.