As long as it is what they class as a valid formation i.e no 2-6-2 or 6-3-1. Or at least that has been the case for the past few years. In some cases only 1 sub will come on even if you don't have a full line up playing that week for whatever reason.
Maradona or Messi?
Xavi or Iniesta?
Eto'o or Henry?
Zidane or Platini?
Stam or Vidic?
McGrath or Ferdinand?
Schmeichel or Van Der Sar?
Baggio or Del Piero?
Robson or Keane?
Cafu or Thuram?