Nimbostratus
After thinking about it further I would try adding an additional self-IP on the GTM/LTM unit and use one for the GTM and the other for the LTM. That way you can add the LTM pair as redundant without conflicting with the IP of the GTM.
The Achilles' Heel is that even with an active/standby setup you still have a single point of failure if the unit with GTM fails. The GTM listener will follow the Active unit so you won't be able to failover to the LTM-only unit without losing GTM functionality. You may be better off running as a standalone or purchasing the GTM license for the second unit.
-George
