A resposta é: não é possível
DRBD dual-primário, iSCSI e multipath: não faça isso!
“Dual-primary” iSCSI targets for multipath: does not work. iSCSI is a stateful protocol, there is more to it that than just reads and writes. To run multipath (or multi-connections per session) against distinct targets on separate nodes you’d need to have cluster aware iSCSI targets which coordinate with each other in some fashion. To my knowledge, this does not exist (not for Linux, anyways).