I have a ranger with archery focus that uses 2 weapons as a back up. When he attacks he uses one action to use the main hand, then I can then use my bonus action to use the off hand, or chose any other bonus action.

But when I give Astarion 2 weapons he always uses both weapons regardless. Which is frustrating when he kills the enemy then wastes his bonus action attacking nothing.

By 5e rules the way it works with my ranger is how it should work, and I would mostly use the off hand if my first attack missed, to try and get sneak attach on it.

Tell me if I'm wrong, but I don't think there is anything broken with Astarion...
Any of my Party members, that I give two Daggers for example, uses Action and Bonus-Action for the main attack to attack with both weapons.
It even explicitly tells so when hovering over the Attack in the Hotbar.

However, is your Ranger by any Chance at or higher than Level 5?
I do not have a Ranger in my Party to test, but when I do that with Lae'zel she can actually attack again with one Weapon, after she attacked with both weapons using the Bonus-Attack Feat for the Fighter which, according to the Wiki, your Ranger also has if it is at or higher than level 5.
If that is what you experience, then you are out of Luck as Astarion will never get that as a Rogue.