[2x2 Tournament] Double Up [SoD]

What if I break the Garrison of Order (GO) to an opponent's Castle, capture it, and my teammate teleports there using Teleport and starts an offensive from there, without breaking their own GO, or if they transfer troops to me, is that a violation?

No, that is within the rules. Moreover, an ally can even buy heroes in the captured Castle without using Teleport. Similarly, on Jebus, when one player breaks the GO and captures the central Castle, the other can teleport there, but they cannot make a direct attack (DD) through the GO.