Yes you can use both products at the same time but not on the same day. When the level of infestation is high, it is indicated to use both products.
I will give some information about Capstar
Capstar is indicated for the treatment of fleas on dogs and cats.
Dosage and administration
Capstar tablets are administered orally at a minimum recommended dose of 1mg/kg
Capstar tablets should be given orally, either directly into the mouth or mixed into a small portion of food and given immediately. Administer on any day when fleas are seen on the animal. No more than one treatment should be given per day.
Special precautions for use
Do not use on animals less than 4 weeks old or weighing less than 1kg.
Effects on fleas may be seen as soon as 15 minutes after administration. Between 95% and 100% efficacy is observed within the first 6 hours and 100% efficacy is reached within 24 hours.
To help prevent re-infestation, the additional use of a suitable treatment to control immature stages of the flea life cycle in the environment is recommended
Nitenpyram is neither embryotoxic nor teratogenic. It can be given to animals at all stages of pregnancy or lactation.
No adverse drug reactions were seen when nitenpyram was administered with other products commonly used in small animal veterinary practice including commonly used flea products, vaccines or antibiotics.
Overdose: nitenpyram is well tolerated. Overdosage up to 70mg/kg was asymptomatic