Do outpost siege abilities stack?
Each Outpost Siege will trigger at the beginning of each upkeep. Example:


You play Outpost Siege, naming Khans.
You play Outpost Siege, naming Dragons.
At the beginning of your upkeep, both Outpost Sieges trigger. 
You stack the triggers in whatever order you choose.
You resolve the triggers in the opposite order.


As each trigger resolves, follow the instructions written on the card.



If you control three Outpost Sieges, then all three will trigger. If you control ten, then all ten will trigger. There's no rule that says, "Only one enchantment with the same name can trigger at the same time."