Togglable fuel cross-feed on decoupler/separators

[Tergiver]

I have on several occasions wished that stack decouplers/separators had the option to become fuel cross-feed capable. It would be nice if fuel cross-feed could be toggled on/off like it can on the docking modules. It should of course default to not enabled so that they behave as they do now by default.

It may seem odd, but there are times (e.g. Kethane rover, Eve lander) where I needed the payload to be empty of fuel after achieving Kerbin orbit, but wanting to use that fuel on the way up.

Right now I use fuel lines to route fuel around the decoupler, but since you can't do this directly if you have tank->decoupler->tank, you have to add some crazy work-around like a small fuel tank that sticks out on the bottom tank that you can route and then again to the bottom tank. In 2x symmetry at least, to prevent unbalancing your rocket.

An alternative is to use docking modules instead as they are togglable, but this is not always possible. One example is where I need to jettison a stage with sepratrons. It's not possible to undock and ignite the sepratrons simultaneously. One would have to add a probe core to the section which is supposed to be debris just to activate the sepratrons. I like my debris to fall back to the surface, so I often do this maneuver at 70.5km, point retrograde, then stage.

[leax256]

for the last problem you could put those 2 action into a group.