What are the possible causes of pain in both feet?

Numerous: Tendonitis, plantar fasciitis, sprain, strain, fracture, etc.

Too little info: This is virtually an impossible question to answer- the list would be book length. You need to be evaluated and diagnosed- a good history and physical exam and follow-up testing is needed here for a good answer.

So many: You need to give some help here, what part of the foot, when does it hurt? What does the pain feel like?