Hand-me-downs...

[Xalioniaf]

About 2 weeks after I started playing FFXI on the PS2, a friend of mine spontaniously bought it for the PC. About half a month afterwards, he quit. He offered to sell me his Vana'diel Collection, but I was wondering that if he's already registered it and everything, if I could install it on my computer and still play it(so I can run my character and my bro can run his :P).

[Intensity]

Yeah, the Software itself can be installed on however many machines you want. But the same POL ID itself can't be active on SE's servers from multiple machines.

For example, I bought my PC version of FFXI @ NA release. I've installed it on 3 different computers since.

[Susurrus]

If your brother is on the same POL ID as you, you won't be able to play together, but you can use your account on any client. Just not multiple at the same time.