Originally Posted by
shefgab
If the Also worth noting, in most countries it's illegal to have two valid passports of the same nationality on your at the same time (when abroad). If the OP gets searched leaving Spain/entering the UK with 2 valid US passports, it might not be a great outcome.
That is not true.
It is worth noting that in most countries with US citizens, it's quite routine for some US citizens to have two US passports in the host country and there is nothing illegal about it. On the contrary, in some such cases, it would be a violation of US legal requirements and even perhaps of host country rules not to have two US passports during their travels to visit other countries for personal purposes.