Loads of them do, given the proximity. Failing that, the Argies (and all the other Latino countries) and Brasilieros converse in ‘Portuñol’, which is a hybrid of the two……….
In the border regions maybe but it's not that widespread, is it? Despite the shared vocab and grammatical structures, spoken Portuguese and Spanish are far from mutually intelligible (the written form is easier to decipher for a speaker of one of the languages).