Matching outfits + matching concertinas. How can you go wrong with that?
These guys are playing a vira, a dance that is played like a fast waltz with heavy emphasis on the first beat. You can hear a guy doing that by beating on a drum in the video.
I've mentioned before that Portuguese players refer to the instrument I call a diatonic button accordion as a concertina. There is a kind of carnival flavour to the sound of this instrument, compared to say either the Hohner or the Guerrini I play. The reason is the tuning. Each note is made up of multiple voices or reeds (2, 3, or 4 depending on the box), each tuned an octave apart. If you tune the different voices just a little bit off perfect, you get a tremelo effect, which is known as wet tuning or musette. Many Portuguese players like their boxes to be tuned "wide-open musette". In other words, they want the maximum amount of tremelo they can get without the instrument sounding out of tune.
The last time I was at my friendly neighbourhood accordion outlet, they had some Ranco boxes which were made with two voices, no switches, loud reeds and musette tuning specifically for Portuguese players. Maybe it's because I'm used to a drier sound, but the wet tuning actually sounded out of tune to my ears.