I own both exact pieces, but I never thought about them as a perfect 2-watch collection. But now that you mention, they are two great daily wearers, with the Aquanaut giving you that extra sportiness, perfect for traveling and vacations.
As s third piece, I spotless either go dressier or sportier, depending on your lifestyle or needs.
I would initially look at the dressier side, you have many options in the catalogue: Worldtimers, Annual Calendars, Chronogaphs... in any precious metal of your preference. Maybe you can add some color with yellow or rose gold.
Enjoy the journey!