I can define a material type in the standard database but can I define more than one for example "glass" and "REE"? Neither ";" or "," seems to work as a divider. If this is not yet possible, could that be implemented?
So I looked at the code and the intention of the define standard material, and the define standard mount fields are treated differently.
In the case of the define standard mount, the same standard can be in more than one mount. So the program expects you to enter the mount name and when it finds a match it only displays the standards in that mount.
The standard material is different. It display all the different standard materials that have been defined by the user and the user checks one or more of the checkboxes to display only those materials.
But one can edit these materials in the Standard application with multiple definitions, for example here, the thorium silicate is defined as both a silicate and an actinide:

And in Probe for EPMA from the Add/Remove Standards dialog one can check this checkbox to see materials that are silicates *and* actinides:

Of course one can also check both the silicate and silicate, actinide checkboxes and then one will see all silicates and also silicates that are oxides:

That's the best we can do I think.