No, Bartleby is right.SGLs are never blocked, MGLs are blocked by any kind of leader.Why?
For SGLs it is for design reasons - you have a fixed chance to get from a specific encounter - researching a tech first.You can maximize your effort in this discipline, but there is a natural limit on how many you can get in a game.So stacking is not too unbalancing.
MGL are caused by the encounter "elite unit winning battle" (with some more restricting factors; f.e. not against barbs) and you can farm them this way, because you can increase the frequency of this encounter.You can have dozends of them in a single game, so one attempt to balance things a bit was this restriction.No more MGL in vanilla/PTW when there is already one.With Conquests, SGLs were introduced...and the game has no strict border between both (just look in the editor, only one leader unit!), so SGLs trigger the blockade, too.It is questionable from view of logic and you might call it a bug, but it works this way.Believe us, it was tested by many players and the result was the same in every test: SGL are always possible, MGLs are always blocked by any existing leaders.