I do not think there is an arragement for this, but in the past the following happened:
Queen Wilhelina married Duke Hendrik of Mecklenburg Schwerin and could add Duchess of Mecklenburg-Schwerin to her titles, as did her daughter. The Duke became Prince Hendrik of The Netherlands, Duke of Mecklenburg-Schwerin
When Prince Bernhard married Juliana he could keep his title Prince of Lippe Biesterfield, but his senior title was Prince of The Netherlands. His wife thus was Queen Juliana of The Netherlands, Duchess of Mecklenburg-Schwerin, Princess of Orange-Nassau, Princess of Lippe-Biesterfield (& her other 45 titles, among then Duchess of Limburg)
Their children became princesses of The Netherlands and Lippe-Biesterfield (Princess Irene presently wants to be called Irene van Lippe-Biesterfeld.
Claus also passed his title, jonkheer (squire/sir) van Amstberg to his sons and grandchildren. It will be interesting to see if Amalia can pass this title or if only Claus-Casimir will be able to (as the only male grandchild) The titles of Mecklenburg-Schwerin and Lippe-Biesterfeld can only be passed in the male line.
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In case of Gabriel and Amalia marrying, according to the old tradition it would be Queen Catharina(-Amalia) I of The Netherlands, Princess of Belgium etc etc, and Prince Gabriel of The Netherlands, Prince of Belgium. Their children would, according to the old system, be entitled to the title Prince of Belgium, but as it could create problems in the future I think that there will be a change in tradition, and they will add a junior title of the belgian RF to their list (maybe Duke of Saxe-Coburg), or none at all.
If Gabriel and Alexia will marry there will be a clash of traditions: 1) the grandchildren of the Belgian kings are still princes while Beatrix styled her grandchildren as counts 2) what about inheriting through the female line? Margriet could pass a title but can Alexia? Honouring those traditions I would assume that the children are Princes of Belgium & Counts of Orange-Nassau.
As both countries have a strong emancipated women I think it could cause a problem if Alexia would become Princess of Belgium and losing her dutch titles (as was customary), so I can imagine that both spouses would keep their own titles, and add the title of the spouse.
But the dutch title-system is such a mess that they can change it completely in the next generation again (just look at the mess there is now, 1 RF and 6 different way of naming them). These things will only be decided upon when such a thing would actually happen, which is not likely (helas).
