But it would be dificult either way. The country with the second child as heir, would protest for not having the first-born in their country to inherit the throne, they would think that they were the losers and less important.
Nowadays, two heirs marrying each other and rule both of their respective countries is "impossible". One of them will have to be removed from the succession line, and dare I say, the women will abdicate (not that I think its fair or not, but the public view of a man abdicate in favour of a women is absurd). In the succession law of Sweden, the heir will loose their right to succed the throne in case he/she marries another heir, so if Estelle were to marry Cambridge Baby (if boy), she will loose her right, not Cambridge Baby.
I'm not sure about the other Monarchies though.
But one thing is possible, is that love between heirs might happen in the future, there's always a probability. I won't say Elisabeth, Amalia or Ingrid, but Christian and Leonor (or with Estelle) could fall in love, or Cambridge Baby (if girl) with Christian, or Cambridge Baby (if boy) with Estelle.
I strongly believe that some heirs might have felt something with each other or one-side crush, but because of the Heir issue, it was just a small crush and nothing more. Victoria was the only female heir at her time, maybe she felt at least a little innocent crush, dare I say, for Felipe? Or in the previous generation as well...
Well I'm not saying Love, just a crush or physical attraction, I've read that all of us have had at least 100 physical attraction with the opposite gender in our lifetime.