Can I use multiple tariff numbers for each CustomsItem object?

Unfortunately, no. You have to create a new CustomsItem object for each hs_tariff_number you need to use.

Also, if you need to look up what your hs_tariff_number, visit https://hts.usitc.gov for your item's Harmonized System code. The Harmonized System code is what you enter into hs_tariff_number.