I wouldn't worry about them expiring or anything like that. I wouldn't want to mix two different active ingredients. Say, for example, you had a product with 2,4-D and a second product with mesotrione. If you had equal parts and you mixed them together, you're applying each chemical at effectively half rate--which might not be enough of either to give a good knock down.

If it were mine, I'd limit different active ingredients to different parts of the yard. Plus 2 on the side yard and Weed 'n Feed on the back, or whatever suits your situation. Be sure to write it down and keep track of what does best for you.

Follow the bag instructions, but I believe that granular herbicides recommend wetting the foliage before application--makes sense as when those chems are applied with a sprayer, the target is foliage coverage. So with a granular you want wet leaves for the granule to stick to, dissolve, and release the herbicide.