Not much disadvantage to 2 OBi100s.
You would have to have an extra ethernet port and an extra place to plug in a power supply module.
Trying to call in on one line and back out on the other (OBi Attendent) could be more difficult if the two services were on different OBis.
You would not have the USB, WiFi, built-in router, or bluetooth ports, tho most people do not use those.
On the other hand, you would gain some equipment redundancy. You could take one OBi100 elsewhere as you traveled while leaving one at home. If you have a lot of ethernet connectivity, it could be handy to have the phone jacks in different places.
I think fax would be the same either way.