There's a few ways you could patch it:
-Simple - Mp-1 in MB-1 loop, MB-1 send 1 > MP-1, MP-1 A > MB-1 Return 1 MP-1 B > MB-1 return 2.
- Better
but you loose half the MP-1 signal ie chan B) - MP-1 in MB-1 loop, MB-1 send 1 > MP-1, MP-1 A > MB-1 Return 1, MB-1 send 2 (use short patch lead) > MB-1 return 2 (this maintains the MB-1 signal and lets you mix in the MP-1 on send 1 IIRC this is the way Peter uses his to get a parallel loop as both MB-1 loops are series (device is "inserted" into the signal path)
- Best - User a small mixer Split the bass signal between MP-1 and MB-1 (Y lead or a/b a+b box), MB-1 > mixer chan 1, MP-1 A/B > mixer chan 2 & 3 (panned hard L and R), mixer L/R > stereo power amp > 2 x cabs (this is the way I ran it with my MP-2 and I used the mixer send returns for Fx unit(s) and blended it all together, also my Alembic uses a stereo jack out so neck PU goes to L chan, bridge PU to R chan; I made a stereo jack to L/R mono jacks lead, neck PU (L) into MB-1 bridge PU R into MP-2, added a little stereo rev and delay with the mixer Fx send/returns, also I used the MP-2 cab sim outs as I ran it into Messa P112 cabs which have a tweeter)