I suppose that you have two instances of Informix on the same machine not two databases.
If you are trying to restore the backup of BKP on BK2, ontape needs some of the parameters in onconfig of BK2 to be equal to onconfig of BKP (rootdbs & TAPEDEV ) otherwise it will give the error of mismatch of rootdbs of ontape backup file and BK2. Further it will use the same devices for dbspaces also as of BKP which is not at all desirable.
So I dont think that you can achieve this by ontape, you better take the dbexport of the BKP and dbimport it on BK2.
Since IDS 9.40 it is possible to resore an instance to another instance on the same machine. What you need to do is a so called redirected restore. THis is done with the ontape option -rename. Check 'ontape --' for the complete syntax. Make sure all chunks have the same size and all chunks are written to a new path.