#1
My stack is currently at my practice space now so I can't exactly test this for myself, but I was wondering if you would get a good volume boost if you plugged one guitar into both inputs of a guitar tube amp using a patch cord splitter or some other similar device.

It seems logical to me that it would, but I'm dying to know if it works for sure.
GEAR:
Vantage Ghost Gibson "The Paul" copy
Fender FM-100H half-stack
Regal vintage 30W class A amp head (handcrafted in Montreal)
#2
most amps, when they have 2 guitars plugged in, make both inputs low gain. You might have more volume, but you will have less gain.
Guitars:
Ibanez RG4EX1
Epiphone SG w/EMG's

Amps:
Peavey Windsor Head w/ JSX Cab
MicroCube (for the sake of practice at college)

Pedals :
Fulltone Full Drive 2 Mosfet
Digitech Whammy
Dunlop Crybaby