Examples

White can deliver double bishop mate with Bd5#.
The bishop e5 on the parallel diagonal e5-h8 guards the escape squares g7 and h8.

White can deliver double bishop mate with Qf6#.
The bishop c4 on the parallel diagonal c4-g8 guards the escape square g8.
One of the bishops can be replaced with a queen, and the pattern remains the same.

If a bishop is replaced with a queen, it is possible that queen and bishop are on parallel diagonals of the same colour. This case would be very rare with two bishops, as one of them had to result from an under-promotion.
Here is such an example:

White can deliver discovered double bishop checkmate with Re5#.
The queen on f5 checkmates the king on the diagonal f5-c8.
The bishop d5 on the parallel diagonal d5-a8 guards the escape square c6.

Related patterns

In Boden's mate, two bishops checkmate the king on criss-crossing diagonals.
In Balestra mate, a bishop gives mate, with support from a queen.