The warlock's Thirsting Blade eldritch invocation cannot be normally used with the (non-warlock) class feature Extra Attack, since both feature explicitly say you attack twice instead of once. But it seems to me that you could use both features (if you have them) when using Two-Weapon Fighting.
Assume your character is wielding two light weapons, one of which is your pact weapon. You take the Attack action on your turn to attack twice with your non-pact weapon using your Extra Attack class feature.
Then, given the Two-Weapon Fighting rules, you use your bonus action to attack once using your pact weapon this time.
The Two-Weapon Fighting rules (PHB p. 195) state:
When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand.
But since your have the Thirsting Blade invocation, and you used the Attack action, you get to attack twice with your pact weapon.
The Thirsting Blade eldritch invocation (PHB p. 111) says:
You can attack with your pact weapon twice, instead of once, whenever you take the Attack action on your turn.
Does this combo work within the rules, or is there any interaction I'm not aware of?