Borrow the experience of IB2007 and enhance the nbackup functionality.
For now, we can't take the "level 0" backup and put the "level 1" increment on it without copying both into new file. And after that can't put on result the "level 2" increment.
I suggest to create level 0 backup as RO database, and allow to put level 1 increment on same file.
Of course, when the DBA turn off RO, incremental backup denied for that database file.