The simplest solution is with two external interfaces defined, one for each ISP. Since you have a clustered environement, each gateway in the cluster requires a corresponding external link for each ISP as you have today. Your question was to use the same external link for two ISP. In that case you have to have different subnets configured for each ISP on that interface in Gaia . Remove IP-address from eth2 and add two VLANs instead. Will have to have someone to confirm this.
You probably want to use this redundancy mode:
Primary/Backup: New connections use the primary link as its ISP. In the event of primary link failure, connections switch to the backup link, and any new connections use the backup link as well. Upon recovery of the primary link, any new outgoing connections begin to use it again while the existing connections on the backup link continue to use it until completion.
There are other things to consider when setting up redundant ISP. Follow this link How to configure ISP redundancy