Mitsubishi Electric MELSERVO-J4 Series Transition Handbook page 268

Part 5: Review on Replacement of MR-J3-DU_ with MR-J4-DU_
(b) When connecting two brake units to a converter unit
POINT
To use brake units with a parallel connection, use two sets of FR-BU2-(H) brake
unit. Combination with other brake unit results in alarm occurrence or
malfunction.
Always connect the terminals for master/slave (MSG to MSG, SD to SD)
between the two brake units.
Do not connect as follows.
Converter unit
L+
L-
Connecting two cables to L+ and L-
terminals
Brake unit
P/+
N/-
Brake unit
P/+
N/-
5 - 67
Converter unit Brake unit
L+ P/+
L- N/-
Brake unit
P/+
N/-
Passing wiring