Thank you for reaching out to Atlassian Community!
It's possible to manage tickets from different next-gen projects in one board, but this will require a third board.
As on next-gen we can't edit the board filter to add tickets from another project, it's necessary to create a separated one, that will be the same as a board from a classic project, but it will be linked to your profile.
First, you need to create and save a filter on Issues and Filters > Search issues, for example:
project in ("Project A", "Project B") ORDER BY Rank ASC.
After that, to create this board, you can click on on the left side > View all boards > Create board > Create a Scrum board > Board from an existing saved filter > Add a name to the board, select the filter you created previously and the location you need to select your profile > Create board.
Then you need to share the board for other users to have access. On this new board you created, please go to … > Board settings > General > Edit filter shares.
On the page that appears, select the groups you need to have access to the board and click Add for each of the groups you select, then click on Save and the board will be available.
Hope this helps!
If you have any other question, please let us know.