Thew DRB tanks have several vertcal pieces with a narrow gap in the middle that prevents sloshing. I added a set of flappers at the rear where the pickups are. Fuel goes in but is hard pressed to leave except for overflow.
Right now, both tanks are pickup from the rear. May change to front one, rear the other. I have return lines to both tanks behind the flapper valves at the pickup points. No selector valve. Don't need one. One set of pumps is all you need. I also have a connector line at the bottom of the tanks that works to keep the levels in the tanks the same. The low pressure pump is like most regular pumps and sucking air won't hurt them if infrequently done.
Remember that if you have AN lines(8s in and 6s out) in the system, you have about a gallon of fuel that will continue to circulate from the swirl tank to the injectors/carb and back again. That should last a few miles, and that only starts when the fuel supply to the LP pump(s) runs out. If you are afraid of the return flow going to one tank more that the other, you have to have vent lines with roll-over valves installed(hope you do anyway). When the level of the one tank over fills, the valve will "float up" and occlude the line making the connector line to force fuel to the "low" tank.