Active signal on non-powered controller with same ground line

It's likely that the signal will "pump" the Vdd rail through the
ESD protection network. Whether that bothers the part, comes
down to current limiting and reliable pin current max rating.
But you can't believe datasheets about that, because such
ratings often are set by powered pin induced electrical latchup
triggering (plus a bunch of sandbagging).

If you can stand it, putting a series resistor that meets those
specs (input pin max current) given signal V and the R will
keep you on the right side of single pin destructive effects.
What might ensue from half-@ss-powering a controller, is
another matter. A series input R, and either natural supply
load or adding some, or ensuring that (say) a master reset
pin must follow your DIP-switch disable (not likely true for
a simple POR, if supply is lifting up) might solve that. Might
think about whether there's an elegant scheme, like DIP
switch controls both a "load switch" (power) and hits the
main RSTb to ensure known behavior.

OK, then how about cutting both Vin and GND lines for the board that I don't want to power it.

Maybe use DPDT switches for each board.

---------------------------------------------------------------------------------------------------------------------


Another situation I'm thinking of:

What if; for example, two boards are powered up, and there are SPI signal on board 1 and board 2 is sharing same signal lines with board 1, would that be a problem too ?


==> I started to disqualify the idea of having multiple controllers on the same board.

Like this board from mikroelektronika:



So this board can program different PIC microcontrollers. But I don't know what the manual says about having two controllers at the same time, whether that is safe or not ?

Or I can just plug in the board I want to work with, how about this solution ?

I'm not interested in communicating with another control board on the same development board for this safety issue. If I want to communicate with another controller, I would put the other controller on a breadboard or another development board.