Is there a double power switch that guarantees that whenever A is on then B is on too (with slight time margin for safety)?
There are probably not switches that will do this, all DPDT switches that I have seen work at the same time. You might be able to use a circuit like this with a high side switch:


Default-on, latch-off, MOSFET power switch

The problem with doing so is when the solar panel voltage dropped it would also disconnect the battery. The best thing would probably be to use a DPDT switch to disconnect both at the same time, to add a delay you might place a large cap on the battery side between the switch and the controller.