Yes Zyrtec and Flonase as well as any other allergy medicines can be used at the same time and will actually provide much better and faster allergy relief.
Since Zyrtec is taken orally and Flonase is a nasal spray they can be used together at the same time and the Flonase can help relieve allergy symptoms much quicker while the Zyrtec Tablet is working it's way through your body.