Nowadays I think it's hard to have a marriage between two heirs.
I do not know if it would be possible to happen, and to happen I do not know how this situation would be managed.
It could be possible for two heir to marry but it would be extremely complicated!
Each would rule in their respective country, because there is no way I can imagine how the public, the constitutions or the parliaments would accept a de facto union between their country.
A consort is expected (very understandably) to be totally loyal to his/her new country. That would present a few problems if the consort is sovereign in another country.
Even countries that have an excellent relationship sometimes have conflicts.
So the solution is for one heir to not only give up the throne but also citizenship and loyalty. That would sure present quite a few problems in the country of the person who gives up the throne! No matter what it would cause a major constitutional crisis, even if the public are happy to get rid of the heir. Changing a perfectly working heir in the middle of the road is always a problem and something that would delight the republicans.
Normally a royal who has married into another royal family would be an ambassador for his/her country. But an heir who given up the throne? And thereby "turned the back" on his/her country? No. - That's a renegade, rather than an ambassador.
Another matter is for a former heir to adjust to the new lifelong role as supporter. Going against what the heir has been brought up to all his/her life.
But what if both heirs gave up the throne? Became ordinary citizens and perhaps even settled somewhere else to live a normal life?
- It would IMO be possible for the former heirs. But it would be seen as an extremely selfish act by the public in both countries and rock the foundation of the monarchy in both countries.
But could it work?
What if:
Amalia and Christian give up the thrones and decides to become ordinary citizens and live in say New Zealand, far away from everything.
Could that be possible and what would be the reaction?
Would another solution work, you think?
(I had to use Christian as there are not that many future male heirs around. Japan would be totally out of the question in the present political climate there and I have to plead ignorance in regards to the Arabic monarchies as well as Thailand.)