Suppose land card A says "when a land comes into play, perform action x." Then you play land card B and it says "when this land comes into play, perform action y." Which action happens first?
It depends on who controls Land A. Whenever a single event results in multiple abilities being put on the stack, the active player(usually the player whose turn it is) are put on first, followed by the nonactive player. If they are controlled by the same player that player may order them however they wish. So if opponent controls Land A, X will happen first every time, if you control both Land A and Land B you can choose which happens.
Because the triggering event(s) occur at the same time both abilities will go onto the stack at the same time. Because both abilities are controlled by the same player that player will get to choose the order they resolve in.
If instead you had say Radiant Fountain and your opponent had Tunnel Ignus then whoever's turn it is not will have their ability resolve first. So if Nate controls Tunnel Ignus and Adam plays Radiant Fountain as a second land then Tunnel Ignus's ability will resolve first.
Because the triggering event(s) occur at the same time both abilities will go onto the stack at the same time. Because both abilities are controlled by the same player that player will get to choose the order they resolve in.
If instead you had say Radiant Fountain and your opponent had Tunnel Ignus then whoever's turn it is not will have their ability resolve first. So if Nate controls Tunnel Ignus and Adam plays Radiant Fountain as a second land then Tunnel Ignus's ability will resolve first.