Keep in mind that Discus need soft water with a ph in a range of 6.5-6.8. They also need about 25% of their water changed every day.
Guppies on the other hand require a higher ph range of about 7.2-7.8 accompanied with a bit of aquarium salt in the water at a ratio of 1 tablespoon per every 5 gallons of water.
Their water does not need daily partial water changes as does the Discuses water.
In theory you could house the two fish in the same water but it would a constant balancing act. Discus should preferably be housed in an all Discus tank.
Listed below is an excellent link to a site that will provide you with ample information regarding Discus care and a link below that which will supply you with information regarding basic guppy care.
Good luck and thank you for your accept.