Can we have two different external VLANS on Active/Active Scenario

I'd actually recommend putting both Traffic Groups on the same vlan, both for internal and external. Traffic Groups are ultimately pinned to IP addresses or Partitions so there's often no benefit to trying to pin them to vlans. It would just add more work both on the LTM and also the switch.

The only scenario I can see where it would be useful is if you're not running BGP and have different IP blocks dependent on different ISPs. Then there would be some kind of DNS management system like a GTM doing round-robin on both IP addresses.