Since theirs 2 inputs does that mean you can plug in 2 guitars at once? if so, are there any disadvantages in doing so?
Yes, and yes. Plugging both guitars in makes both guitars sound bad. So don't do it.

BTW, it's not really made for that.
when both guitars are plugged in they will basically sound the same, so yeah, it's pretty annoying
Ibanez JS100
03 Squier Strat
1980 Ovation Matrix Accoustic
Peavey Valveking 112
Washburn T-14 Taurus (Bass)
SWR Working Pro 100 watt bass amp
how much is a valve king

Ibanez GSA 60
Ibanez RG4EX1QM
Metal Muff
Jackson DKMGT
GK Backline 210
Vox VT50
Crate XT15R
Ibanez SR400QM
Ibanez GSR 200 bass
Crate B15 bass amp
Lots of Monster cords
i got it for 500 plus shipping of ebay, its amazing seriously. I had to sell my Marshall tsl halfstack to buy a car so i bought the valveking. It pwns the marshall, nuff said.
No. There's usually a high input and a low input, where the low is better suited for cleans/mild overdrive while the high is better for crunch/distortion. As to whether it damages the amp, I've no idea, I've never seen anyone stupid enough to try.
I'm a communist. Really.