It sounds like there is an IP address conflict. You and your wife may be sharing the same static IP address. When two machines with the same IP connect to a network it is common for both connections to go down.
I would start with a reset of the defaults on the MiFi to set it back to DHCP. Then I would review the individual IP settings on your two personal devices. Make sure they are set to automatic IP addressing. If you google your "OS and automatic IP" you should be able to find guides on how to confirm and complete these steps.
Then try everything again and see if you can hold the connections.