As mentioned in another thread, Mp3Tag does not actually store a double backslash in the file, the double backslash is only how Mp3Tag displays that a tag has multiple values.
If you can use ID3v2.4, then you get proper multi-value support for tags without having to specify any separator. If you stick with ID3v2.3 use the separator that is supported by your software, both semicolon and slash are common choices. If the software does actually support multiple values. A lot of software doesn’t and only displays the tag value as is, in this case the choice of character boils down to personal preference on how you want to have it displayed.