I am not sure of the point of SN2173 as the first arrival of the day at Manchester, as it is the same time as the flybe flight.
It's always possible that the BE and SN relationship will be strictly a wet-lease deal without codesharing, in which case SN would be competing with BE on MAN-BRU
Sounds odd, but it does happen in the US where Republic Airlines has operated routes for Delta in competition with Republic owned Frontier Airlines