I've used both Captain's and Flagship, but not in conjunction with each other. At this point, I prefer the Flagship, but I've found that you have to thin it a bit with their "T-10" thinner to get it to go on smoothly. (Since they both use the same thinner and are made by the same company, I would be 99% certain that they are compatible.)

As for sags, as the other's have pointed out, multiple very thin coats will have less tendency to sag. Another "trick" I've used is to prop the boat onto its side while varnishing (i.e., get the surface as close to horizontal as possible while it drys). This may take a bit longer, but will go a LONG way toward eliminating sags.