Citra is not really citrusy despite the name; rather, it is tropical and full of mango and lychee.
Apollo is citrusy and dank. It's very potent and reminiscent of marijuana.
Use them both sparingly in the boil or things have the possibility of getting too catty, harsh, and off-putting.
You can use both in equal amounts if adding to the dryhop, or even a hopstand around 165 F, where the alpha acids are not being isomerized.