If your using 2 Mifi's from the same provider....eventually the same IP will come up on both devices (not at the same). You should still log IP changes at all times.
well, maybe I will be burned alive for saying that, but I don't think it is useful to log your IP because you potentialy use the same IP as everyone who use the same provider. You could use 1 mifi device or 10 , it would be the same. It would be more relevant to work on the other linking methods (cookies, browser fingerprints..) to appear as a totaly different seller. I think they can't link your accounts based ONLY on the same IP.
I may get burned alive for saying this myself. I think that if you use 2 mifi devices from the same provider that you may be paying for something that you dont really need. Now keep in mind that i am talking about a major carrier in the US. I use a single 3g mifi on all of my accounts. I do keep track of IP usage on each login. I have found that it is a rare occurrence to gain a repeat IP when shutting down mifi and restarting between switching accounts. Usually the repeats happen only when switching to fast (shutting mifi down and turning right back on), i have found that waiting a couple of minutes before restarting the mifi nearly completely eliminates repeat IP addresses. Personally i use a mifi for 3g, and cell tethering for 4g. The cell tethering comes in useful when i need to upload faster, or when net traffic makes the 3g mifi just irritatingly slow. Keep in mind that i log the 4g cell IP address in the same spreadsheet as my 3g mifi. I have considered using a second 3g mifi, but only as a way to log into two accounts at the same time, or for some streaming or regular internet browsing at the same time. As of yet i just can not justify adding another 3g mifi at this time as between the single 3g mifi and the 4g cell tethering I have all the net access that i need. One thing i have been looking into lately is combining 2 3g devices in order to double the speed. As of right now all the research i have done points that it may indeed be possible, however it seems some limitations may make it so that it is not beneficial at all (jury is out).
2 mifi devices 
jury is out).
