You must be holding both weapons when you attack to use the Combat with two weapons, and attacking with a big sword requires both hands.
You have the Dual Wielder feat, so it does not matter that neither of the two weapons is light. So far all good, however:
The rules of combat of two weapons establish:
When you take the Attack action and attack with a light melee weapon what do you have in one hand, you can use a bonus action to attack with a different melee weapon. what are you holding in the other hand.
There are two problems here.
First, and more simply, when you attack with a big sword you are not "holding" [it] in one hand & # 39; – It is true that you can keep a big sword in one hand, but not while you are attacking.
Two hands. This weapon requires two hands when you attack with it. This property is only relevant when you attack with the weapon, not when you simply hold it. PHB
Secondly, although it is not completely explicit, it is generally understood that TWF requires that you already have both weapons, when you take the action of Attack with the first weapon, to have the option of Bonus Action attacking with the second weapon. To obtain consensus evidence on this decision elsewhere, see these other related questions, here and here.
To attack with your big sword you need to use both hands, not just one, and you can not hold your long sword with the "other hand" while doing it. Therefore, you do not meet the requirements of the combat bonus action of two weapons.