First rental contract (“résidence principale”) in France started 1st of december 2018. The second - 1 Feb 2019. So there should have been a be a corresponding Tax d’Habitation for the first contract arriving through the post in Dec .
Which address would it arrive to? If that would be the 1st address, then what happens if there is no one to pick the letter and pass to ex-residents?
Since there is no access to the 1st address, is it possible to find the tax amount & pay it online (if one doesn’t have a tax code in France)?
What form of rental contract? If it was just for 2 months dec ‘18 - feb ‘19 was it a holiday let contract? You say résidence principale, which suggests not, but perhaps you meant it was your only residential address and not that you informed the owners that you had no other principal residence? (Holiday let contracts are only supposed to apply to people with a permanent address elsewhere, so TdH doesn’t apply)