When such Plots are revealed at the same time, how many Intrigue Challenges may each player initiate?

One, two or none?...

The answer is none.

In this game, the word "cannot" is something of a gold standard or "trump card." If you "cannot" do something, that takes precedence over everything else. With Siege at Riverrun saying that the player cannot declare intrigue challenges at all, it doesn't matter how many opportunities to declare them the player is given. He is always stopped from following through.

Keep in mind that Siege at Riverrun only stops the person who revealed it from declaring intrigue challenges. It has no effect on any other player. So if I reveal it and you reveal Breaking of Oaths, I get 0 intrigue challenges, but you still get 2.