Have a look at the basic setup manual v1.2 it holds all the info regarding the MAGG bonding for 2 MHO's in 1 site.
But at first glance I see where you went wrong, all eth1-xxx interfaces are on MHO1 while all eth2-xxx are on MHO2 so you need to build a bond between the eth1-Mgmt1 and eth2-Mgmt1
The way to build this would be like this procedure (I just found some typos in this part, sorry):
add bonding group 1 mgmt
set interface eth2-Mgmt1 state on
add bonding group 1 mgmt interface eth2-Mgmt1
set bonding group 1 mode active-backup
set interface magg1 ipv4-address 1.2.3.11 mask-length 26
set management interface magg1
delete interface eth1-Mgmt1 ipv4-address
add bonding group 1 mgmt interface eth1-Mgmt1
set bonding group 1 primary eth1-Mgmt1
Regards, Maarten