Keep the pass from yesterday, and use it today. But you have to actually spun the pass from a gym yesterday.
Then after you used the pass from yesterday, spin a gym for today's pass.
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There is no way to get 2 free ones every day, just 2 free ones in 1 day.
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As ^these 2 say. Always spin the gym after you do a raid. I personally like to hold on to my pass from yesterday so I can do 2 a day if I happen across others doing a T5 or T4. Otherwise I just solo a T3 every day.