VS Configured on the GTM Pool may not require to be the VS on the LTM always. You can also have direct servers (Generic Hosts) to be configured as VS in the GTM Pool.
When you use the LTM's VIP's as the VS in the GTM Pool, its recommended to have the server created in the GTM with product BIGIP (If Iquery is setup). Else choose Generic host.
Creating new servers
create gtm server 1.1.1.1 datacenter "UK" monitor tcp_half_open addresses add { 1.1.1.1 } product generic-host virtual-servers add { 1_1_1_1_80 { destination 1.1.1.1:80 monitor tcp_half_open } }
modify gtm server 1.1.1.1 virtual-servers add { 1_1_1_1_443 { destination 1.1.1.1:443 monitor tcp_half_open } }
Creating new gtm pool.
create gtm pool test_newpool load-balancing-mode round-robin ttl 30 members add { 1.1.1.1:1_1_1_1_443 }
Creating new wideip
create gtm wideip test.wideip.com ipv6-no-error-response enabled pool-lb-mode round-robin pools add { test_newpool }
As you can see the Wideip is set up to resolve to 1.1.1.1, but its monitoring the port 443. If port 443 is down, the wideip is down. If port 80 is up/down, wideip doesn't bother.
The GTM gives out the IP of 1.1.1.1 but its your machine which controls the port.
So if on the destination end, the port 443 is not running but just 80 is running, the traffic would fail.
Here the destination could be VS on the LTM or Direct servers too.
So if VIP 443 is down, traffic fails. If user traffic is on port 80, and VIP 80 is up, traffic passes.
When you say both bigip systems, Is your setup having Iquery installed.