1 or 2 designated journeys by using Airport Express Travel Pass

The two designated journeys means the journeys to the HK Station in Hong Kong Island and Kowloon Station from the HKIA.
If you bought the card with two designated journeys, it is just one card. If you bought the two single journeys seperately, you will get two cards.
If you have the Airport Express Travel Pass for single journey, it can be used on the one journey from HKIA to HK Station or Kowloon Station and three consecutive 3 days on MTR.
If you have the Travel Pass for round journey, it can be used one journey from HKIA to HK Station on in one week and then the three consecutive 3 days on MTR and then the other journey from the city down town area to HKIA the next week.

I think if you are still not clear, it is suggested to find more detail information on the official website of HK MTR.

Thank you for the reply! One more clarification question. It seems for what I need, having two separate 1 journey ticket will work for me. Will 1 journey allow me to have roundtrip on the airport express from the same station? for example, if I am leaving from HKIA to Hongkong Island, and need to be back to HKIA the next day, will the 1 journey airport express travel pass work for me?

You mean you will buy two 1 journey travel pass tickets?
If so, the 1 journey travel pass will work for you.