A: On most amps, the high input runs straight into the amp’s preamp section and offers the most gain. The low input is often buffered down by 6dB or so to provide more clean headroom when using hot pickups. Try each and see which works best for the sound and gain you want — you won’t hurt anything by using one versus the other.
In most amps, you can plug into both inputs simultaneously, though some manufacturers, notably Orange, recommend not doing this. In most cases, when you connect to both instruments simultaneously (for example, running in two guitars at the same time), the two jacks will have equal gain.