We are still considering when/how to implement this. Currently, this is the best workaround:
- Select filter 1.
- Select all papers in the current list.
- Assign a new label to them (may take a minute or so).
- Select the new label, then apply filter 2.
Note that if you later add papers, the label may not perfectly match filter 1 and may have to be updated if you want to do this type of filtering again.