Are your projects just for building a collection of files or do you really make use of the project features?
Project features are Find/Replace in Project Files, project related wordfiles, project related templates, CTags for a project, a function list view showing all functions found in all files of a project, project specific workspace, etc.
Having multiple projects open at the same time would make it very difficult for the IDM developers to offer all those project related features and for the users to make use of them. That would require a complete redesign of project management within UltraEdit and would require further a different use of those special project features by the users. Especially if an opened and active file belongs to more than 1 project currently opened it would become very difficult to determine with which project those special project features should work now. I have projects with shared files between the projects.
If you are using projects just as files collection, you might try using the Lists tab and setup several lists with the individual files for quick opening.
Another suggestion is to configure a large project which contains several groups (not folders) with the names of your current projects and add to the groups the individual files and folders. That's similar to a solution (= a set of projects) without special features supported by real solutions.
I wrote here what is supported by UltraEdit for Windows which I use and therefore please forgive me if some features are not available in UltraEdit for Mac. On Windows it is easy to start a new instance of UltraEdit from within UltraEdit and open in this second instance another project. That's what I do if I really want to do something on the files of two projects simultaneously which I have not added to a solution in UEStudio which I usually use for working with projects.