USB modem and high availability

We have two ASG 220s (8.306) set up in hot standby HA mode. I would like to attach a USB 3/4G modem to them to be used as a last resort if our other two high speed connections go down. How does HA deal with USB modems? I assume I need to have two modems, one plugged into each ASG, but won't they each have different IP addresses? Just wondering if anyone has done this before and if there is anything special that needs to be configured.

Thanks!