WMA tags are a little special when it comes to supporting multiple values. There is a list from Microsoft that details which fields actually support multiple values
However, not all are supported by Mp3tag (e.g., the WM/UserWebURL field is a binary field that currently isn't).
For a complete list refer to the official Mp3tag mapping, where supported fields have an entry at ASF/Windows Media, e.g., WM/AlbumArtist.
As for support for WWW in ID3v2, it internally maps to WXXX which is defined as
This frame is intended for URL links concerning the audio file in a similar way to the other "W"-frames. The frame body consists of a description of the string, represented as a terminated string, followed by the actual URL. The URL is always encoded with ISO-8859-1. There may be more than one "WXXX" frame in each tag, but only one with the same description.
This means, you can only one WWW field to an ID3v2 tag (e.g., in MP3s or WAVs ID3 chunk) with an empty description. If you intend to use more than one of such fields, you need to use a different description.
In Mp3tag, such descriptions can be added as part of the field name, e.g,
WWW DESCRIPTION = Field contents