You can't combine two separate bookings, but if you have both PNRs/tickets with you when you check in the agent can certainly tag your bags right through to MAN.

With a bit of luck (and a competent agent) they should be able to issue both BPs too, if that doesn't work you can always collect your second BP via OLCI, on the app on your phone, or indeed at Flight Connections after you land at LHR.