The two products can be used together as they have totally different active ingredients.
Program plus is a preventive medicine against fleas and heartworm. When I say preventive it means that it will not kill adult fleas or ticks or adult Dirofilaria immitis. It only fights with immature stages of fleas and heartworm. The protection against heartworm is good but for fleas an insecticide to deal with mature stages is needed, so the use of Capstar is highly recommended.
ProgramPlus tablets have the active ingredient lufenuron and milbemycin oxime. Lufenuron is for the prevention and lasting control of flea infestation and the treatment of flea allergic dermatitis in dogs. Lufenuron is effective against the dominant flea species Ctenocephalides felis and C. Canis. Milbemycine oxime It is active against mites, against larval and adult stages of gastrointestinal nematodes as well as against larvae of Dirofilaria immitis.
To be fully effective the tablets must be administered together with food, eg, added into a portion of the daily food, hidden in pieces of meat, etc or administered by mouth after feeding. After administration, the dog should be watched for several minutes to ensure that the whole dose has been swallowed, it is very important if the product is used for Heartworm prevention.
Capstar tablets for flea treatment are given on the day you spot fleas on your pet. They kill 95% of fleas in the first 6 hours and the remaining of them in up to 24 hours. They can be given as often as every day but there is no point in doing that if there is no need to. Fleas from your pet are killed but it has no effect on immature stages of fleas (for which you can use Program) and the adult ones found in the environment, so different product need to be used if you want to control these aspects as well. So once your pet is treated if the environment wound be flea free it will take around 7-10 day for a new generation of fleas to develop. But as the environment is full with different stages of fleas you have to check your pet regularly and administer Capstar on the day you see fleas on it.