How are the deathrattles of two Sylvanas dying at the same time resolved? [duplicate]

Order of operation gets very complicated in Hearthstone. If you feel like reading a novel, check out Gamepedia's advanced rulebook page.

In the case of minions dying at the exact same time (for example, when a Sylvanas attacks another Sylvanas), their deathrattles activate in the order that they were summoned.

From http://hearthstone.gamepedia.com/Deathrattle:

Note also that when multiple minions are killed simultaneously, their deaths will be resolved one at a time, in the order in which they were summoned.

For example, if a Sylvanas Windrunner attacks another Sylvanas Windrunner, destroying both minions, the Deathrattle of the Sylvanas which was summoned first will take place first, followed by the Deathrattle of the Sylvanas which was summoned second. Order of minion summoning can therefore be extremely significant.