If you are signed up for the double, you will get two bibs and two chips, one for each race. You need to first run the short race and finish. Then you need to change your bib number and your chip number, and then wait for the start of the long race before you start. Do not take both chips with you during any of the races as this will mess up your results.
The ideal thing to do with the second chip is to just not pick it up until after you finish the short race. The packet pick-up for the long race does not close until the race starts. So, you should pick your bib and chip for the short race, run the short race and then pick up your bib and chip for the long race.
If you run the races back-to-back without waiting until the start time of the second race, you will not get an official time for the long race and you may mess up the scoring when you cross the start line and finish line the second time.