Flying into Beijing Capital and out from Daxing International

Yes, of course you can. Daxing airport is perfectly fine for visa free transit in the same way as it is Capital airport. When you are flying on this scheme, you are also allowed to mix the airports within one transit area. Also, you should just take care that it is not 72, but 144 hour visa free transit in Beijing because 72 hours were extended to more generous 144 hours years ago. You are good to go.