I want to have the same RMAN backup to exist in disk and tape simultaneously. I have used

run {
allocate channel ch1 type disk;
allocate channel ch2 type sbt;
backup database;
}

But what I got was datafiles are splitted between to backup sets. One set on channel ch1 is stored in disk and another on channel ch2 is stored on sbt. How to have both disk and tape having the exact same backup? Tks.