i dont think that muslim can have concubines as it will lead to zina'..

but yeah muslim can have up to four wives..

and there are strict rules if you want to get 2,3 or four wives...

and the rules are (correct me if i am wrong my muslim frens)

1) U must get the PERMISSION with your first wife first.

2) U must treat your wives fairly.

3) Can afford, which means if you think that you can take the responsibility..then you can go ahead..but,must abide the rule no. 1 first.

So if ur first wife doesnt allow you get a 2nd wife..then its haram...  

but then again...paham2 saja la...