If we are talking about mirroring in the hardware sense, really writing both USB sticks with your ESXi instance the configuration would need to take place within the RAID BIOS. You would need to determine if such a thing is possible with your system. Software RAID within ESXi is not an option for a number of reasons.
This leaves you with asynchronous methods. You may be able to mount the second USB device from within ESXi and copy the appropriate configuration settings from the active to the passive USB but USB support from within ESXi is limited due to USB passthrough. I don't think you want your script enabling and disabling the USB arbitrator as mentioned in this link http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1036340. Doing that regularly may have unintended consequences for your guest VMs.
What to do then? I would recommend using the CLI to backup your configurations regularly using a mechanism like the one outlined here: http://www.virtualizationguru.org/2012/01/backuprestore-esxi-5x-using-vicfg.html Keep a fresh ESXi installation on a USB stick handy and keep it updated regularly from your backups. As regularly as possible, replace the backup key you keep in your servers any time you make a significant change and rely on HA or FT for your host resiliency instead of just multiple boot devices. I've seen the primary USB device fail such that the BIOS won't attempt the second device so it isn't foolproof anyway.