If the sheep are actually wormy (or flukey) then hopefully folks would worm (and/or fluke) first and then Heptavac a few days later. We did this this last time as a few were showing symptoms of fluke and we didn't think they'd manage a good response to the Heptavac if they were weakened by the fluke. Plus if there is a lot of fluke, then the flukicide itself can make them feel more poorly for a day or two while it does its work.
But if the regime is for routine worming and fluking, then I don't suppose it would matter which order it was done in. Or whether they were done together, come to that, in healthy worm- and fluke-free animals.
Normally we do do everything together, because it's better than risking that something intervenes and we don't get to do the second treatment as soon as we'd like. Plus on our farm, there is inevitable stress involved (on the sheep and us too!) in moving all the batches along the roads and to the farmstead, and back again, so it's usually better to do that once only.
But if, as did happen with a couple of batches last year, we feel they are under the weather, then we deal with that first and vaccinate when they are back to full health.