Is it possible to leave one country via land border using one?
Absolutely and for dual (multi) nationals generally required! Most countries require their citizens enter and exit using their passport from that country. A US citizen is required to enter and exit the US with their passport. An Uruguayan citizen is required to enter and exit Uruguay with their Uruguayan passport. I heard stories from Americans who gave birth in Uruguay. They got their baby a US passport. When they attempted to leave Uruguay (with their baby) they were stopped by Immigration and refused exit (for their baby). They had to get their baby an Uruguay passport and then were able to leave without any issue. However, to enter the US their baby had to use their US passport. Not only because the US requires it, but an Uruguay would require a visa. The US would not issue a visa to an American. I also understand that Boris Johnson was denied transit through the US on a trip to Mexico as he presented his British and not US passport. He was returned to London, while he family was able to proceed onward to Mexico. Now, some countries may not pay too much attention (regardless of the law). If you are entering a country you are not a citizen of, then you can use whatever passport you like. For example, if one required a visa and the other not, obvious you would chose the no visa passport. If you enter a country with one passport and leave with a different one, it may result in hassles, confusion or problems. I once entered Argentina using my Uruguayan ID and then attempted to leave using my US Passport. The immigration officer was not very pleased, and scolded me with something like the next time you come to Argentina to go to Asia, USE YOUR PASSPORT!. He also returned all my documents without making any stamps or computer entries. When I returned, I entered Argentina with my US Passport and then left using my Uruguayan ID. So, I suspect that I am in their database as having overstayed well over 10 years now! Once when leaving Colombia, immigration asked me where I was going to. A question they had never asked before. My response was San Jose, Costa Rica. No, in the United States? I said, I have no plans to return to the US. I could see a list of US citizens on his screen and he had to select one to continue the process. As we were discussing it, I pulled out a receipt from immigration in Costa Rica that I had applied for residency there. He entered something and different list of cities came up, which has San Jose, he selected that and I was on my way. That never happened again. However, while I was waiting for my flight, I noticed that my entry stamp (not a stamp, but dot-matrix printing) they had typo-ed my passport number. So, when I was leaving, they had no record of my entry. The hassles can be much greater though. A Colombian left Colombia (the first time) and somehow missed the immigration checkpoint entirely. No stamp, no record in the computer. All was fine, until she returned to Colombia. For a Colombian to enter Colombia, who never left Colombia, well that is a big deal! Ultimately she had to pay a fine (I think about US$100) and obtain proof from the airline that she actually left!