This is my understanding.
MXes in HA works in Active-Passive mode. We need a single License for this functionality.
This means at a given time only 1 MX shall be in service. All the traffic needs to flow via the active MX.
The traffic flow may utilize both the Links (ISPs) using Load balancing and Internet Flow feature.
If you have SD-WAN you have further options to segregate traffic.
Failure of MX / ISPs shall swap the roles (Primary / Spare) of devices.