Cannot connect two W10 VPN Clients from the same remote network (VPN Client Static IP?)

[ultralame]

I am new to this, we are a small company and I am barely up to speed on all the terms we need here.

The Fortigate 60E is set up with an L2TP/IPsec with shared key.  That system is located at our headquarters in KY.

I am in California.  I want to connect two windows 10 clients.  Both are on my home network, which is behind a Ubiquiti USG router, which in turn is in passthrough mode behind an AT&T Fiber modem/router.

I can connect either W10 client without an issue.  (I have separate VPN usernames for each).

BUT if I have one connected and then I connect the other, the previous one is kicked off.  That is, I cannot seem to connect both at the same time.

I was able to connect both simultaneously by connecting the first, then connecting to the Fortigate VPN through ANOTHER VPN.  So maybe it doesn't like two connections originating behind the same MAC address (presumably my Ubiquiti Gateway)?

Any help would be appreciated.  I am working with our "admin" in KY who is also not very well-versed with these products.

BONUS:  If it's an easy answer:  How can we set a static VPN IP for one of these clients?

[sw2090]

https://forum.fortinet.com/tm.aspx?m=73864 might be a solution to this too...