I'll search on AFA or some sites about our rules now. The fact is that is illegal for players to play in two differents teams between two transfer window. They're exception, when transfer window is still open and Apertura or Clausura has already started (so stupid we are that transfer windows ends at first week, second for free players), but in general, let's say Defensores loans "Andres Aimar" from River for 3 months, August, September and October ( this Apertura, for Primera A and Primera B Nacional, goings since August to December) he will be a Defensores players for all the mini-season, even if he returns to River. In November, when he's supposed to return according the loan contract, he can't play for River because he's a Defensores players on the AFA registers.
Well, that's the way I know, I'll search from now on the rules and post it here, if I don't found this, I'll mail tycsports (TV signal in Argentina that has the rights of the Tourneys) asking the rules since they clarified a lot of people about different rules. If you wanna go ahead, www.tycsports.com, then search in "Programas" the show "Estudio Futbol". In the main page of this show, there are some journalists, ask who you want about this (I'll recommend Alejandro Fabbri since he is the conductor, a ppity an ex-referee has no contact, cause he knows ALL about our rules)
Well, hope I could found that in time, if not, well, till FM6 I suppose