Multiple nationalities

My understanding is that Belgium and the UK allow multiple nationalities but the Netherlands does not.
My wife is a Dutch national and a resident of Brussel for over thirty years, should she choose to acquire Belgian nationality she would have to relinquish her Dutch nationality.
I am in the process of requesting Belgian nationality which allows me to keep my British nationality.
Simply, if you are a Dutch national you can not have another.

There are certain exceptions however.
Exceptions to the renunciation requirement

In several situations you are not required to give up your original nationality.

In some countries you automatically acquire the nationality of that country if you are born there. And it is up to every country to decide when their nationals lose their nationality. Greek and Iranian nationals, for example, cannot give up their nationality: it is not legally possible. In Morocco giving up your nationality is not accepted in practice.
If you are married to a Dutch national, you may keep your own nationality. The same applies in the case of a registered partnership.
Refugees who want to be naturalised are allowed to keep their original nationality. This only applies to people who are recognised as refugees in the Netherlands, Aruba, Curaçao or St Maarten.

I would support what Mark says.
However, I am not sure why you want Belgian nationality if you already have Dutch nationality.
The reason most British people are getting Belgian nationality is so that they can continue to live and work in any EU country. If you already have Dutch nationality then you are secure and have full EU rights.

I was born British and took Belgian nationality. There was no question of me having to give up my British nationality.
If you take Belgian nationality there is no way that the Dutch will automatically know, but I agree with WEZEMBEEKWANDERER. Why would you want to?

The British allow dual nationalities, so there is no issue there. However the Dutch are different.

Given how efficient the Dutch are about everything, it's perhaps not surprising that the government has a really easy to read and very comprehensive guide to nationality here;
https://www.government.nl/topics/dutch-nationality

Specifically the page on loss of nationality is here:
https://www.government.nl/topics/dutch-nationality/loss-of-dutch-nationa...