Some kind of adaptor.....

That works like a computer USB hub = Like you plug it into your amp, and you can put more guitar into it.
Is there anything like that that you can buy like in a shop ?
I am just wondering...

I am assuming you are speaking about a 1/4 or 1/8 in. input on USB/Firewire.
The cheapest I know of is the iMic
http://www.griffintechnology.com/products

Ah here, on sweetwater.com you can find a bunch of Audio USB interfaces,
http://www.sweetwater.com/c695--USB_Audio_Interfaces

There are ones with multiple inputs for multitrack recording, as well as higher quality, bit rates, etc.

A friend of mine has the PreSonus FireStudio (about $700), works well, but that is for Firewire.
here's Sweetwater's Wirewire selection,
http://www.sweetwater.com/c683--FireWire_Audio_Interfaces

i use a lexicon lambda usb audio interface...its really awesome...

these all sound like computer stuff,i shoulda been more specific;
I used a USB hub as an example, because yo plug it into one port, and you get additional ports. With wat I am talking about with an adapter, you plug it into one input, you get more inputs. This is plugged into your amp, then you plug a patch chord into the adapter and into the guitar, if this makes sense, sorry

*and i am not talking about like just recording, just for random playing like so you could plug like more then 1 or 2 guitars into your amp..

oh i get it...your amp should have either two inputs already, or an input and an effects loop-which you can use for a second guitar. but i wouldnt recommend playing more than one guitar through anything less than a 100watt amp.

Agh! This is why we should reread our posts!
I am with guitarslinger on this. Don't put more than one input, even if it allows it. You have two guitars, and each of their outputs will be competing for each other. What you find is one guitar will be MUCH louder at times, then it changes to the other, and you play this game going back and forth again and again.
I never tried adapters per se but I'm sure it acts the same(since it depends on the amp, and now you would be forcing two signals onto one input instead of two.
If you need a second input(for whatever the reason) you could always buy a second amp and buy chord that splits or buy a pedal that splits the line for you.