This was a tidbit that came out with the map. One of the nodes within the Freljord region tells us that Ashe and Tryndamere were Oathbound.
The word Oathbound, as opposed to Marraige, is an interesting one to think about. Marraige, at its origin, was a proprietary transaction. Oathbound lacks the same connotation. From what we know about the Freljord, it’s a hard place. I imagine the concept of “giving someone your oath” is a serious one.
I want to conclude by stating that it seems the relationship is one for survival/alliance. Much like in the Pre-Retcon lore, this is an alliance forged out of necessity, to merge two powerful groups into one.