When I order prototype boards (in our local PCB manufacturer) I put several PCBs together, but I don’t assume it will be their panel. They will put my PCB together with others. So I even not consider that it will be able to make v-cut through my PCB and then through PCBs ordered by others.
I order it as one PCB, but as you have a right to make milled openings in your PCB I use it to divide my PCBs. I just left 2mm space between them with bridges with mouse bites. See my biggest example here: