Tested on an orc warden.
Bar 1 one holds 'Growing Swarm' on slot 5
Bar 2 holds 'Travelling knife' on slot 4
If I cast 'Growing Swarm', the GCD indicator for 'Travelling Knife' disappears. Leaving me to wonder how much time is left until I need to recast.
The same thing happens if I cast 'Travelling Knife' on Bar 2. It removes the GCD indicator for 'Growing Swarm'.
The Scripts used for Travelling Knife are 'Bleed Damage', 'Lingering Torment' and 'Vulnerability'.
If you've got trouble understanding what I mean by all this:
/script ZO_ActionButtons_ToggleShowGlobalCooldown() stops working in relation to these two abilities if both are in action.