tarrangar
Fat arctic chicken
Yeah but that's for kings, neither Theon nor Valaena is a king, which mean there's a higher authority namely ours, to enforce their Wills.Landed nobles generally do not marry into each other's lines directly though. In the rare instances when a woman rules in her own right she tends to marry someone low in the line of succession and have him take her name. Also Wills do not actually mean much by Westerosi precedent. Look at what happened to Viserys the First.
So all in all, I just don't see allowing 2 ruling people to marry, and just pass their fiefs down to their first 2 children, as something that will bite us that much in the ass, the rare asshole that will try to use it for a powergrab, is the kind of asshole we would want to deal with anyway.