I travel back and forth between two countries and have a different SIM card and a phone number for each country. How can I use two factor authentication if I cannot always be reached at one of those numbers?
Two factor authentication only uses a trusted phone number when you ask it to. It automatically sends two-factor requests and codes to trusted devices, not to trusted phone numbers. If you don't have a trusted device and need to have the code sent to a trusted phone number, you request that at the time that you need the code and you specify which trusted number to use. You can set up multiple trusted phone numbers and they can be any number including Androids, landlines, friend's or relative's phones.
See information in Two-factor authentication for Apple ID - Apple Support
Two factor authentication only uses a trusted phone number when you ask it to. It automatically sends two-factor requests and codes to trusted devices, not to trusted phone numbers. If you don't have a trusted device and need to have the code sent to a trusted phone number, you request that at the time that you need the code and you specify which trusted number to use. You can set up multiple trusted phone numbers and they can be any number including Androids, landlines, friend's or relative's phones.
See information in Two-factor authentication for Apple ID - Apple Support
Cannot access phone number for 2 factor authentication when traveling
