they don't cooperate (hm...they do at all but they don't know it)...when big player wants big amt, they divide it to small parts and usually dealing it live with another banks via ebs or dealing 3000 - they don't put classic order because they don't want to be seen, they call for 50mio and then you quote the bid/ask for them and he ll sell or buy from you and then the desk will make the callout where they divide that 50mio to smaller parts and deal usually that amount back with few others banks...this callouts usually ends with a loss in book, but banks are doing it even that, because they have the FLOW and they know what the big player is going to do - yes you need some skills to decide if he is only profit taking or so but it's usually not so hard to decide, and then what will you do? you will just buy 5mio with him and watching what's gonna happen, the other banks where he bought another 50mio(part of big cash), they will probably bought too few milions...and that's the hidden cooperations of banks on interbank market, so they will accumulate another volume to push it higher...but i've never met with sth like the two banks agreed that they will buy xxx in certain time...