Brazil and Argentina to start preparations for a common currency

Will the US dollar become a secondary currency for the area and have less value?
Will the US dollar become a secondary currency for the area and have less value?

That is a profoundly important question, and the answer is unknowable.

Certainly the actions of the US government over the last decade have done much to call the value of the dollar into question, and the grossly, recklessly irresponsible behaviour of the Biden regime in particular has accelerated that process of decay. Sanctioning half the planet has forced many nations to seek non-USD solutions to settle international trade. The freezing and threatened seizure of Russian assets, in particular, has done much to cause other nations to doubt the wisdom of placing their assets within easy reach of our dear old Uncle Sam. This is simple enough for a 5-year old to understand; if people trust you to safeguard their toys and you then steal those toys, they won't trust you anymore. Your reasons, (or alleged reasons), for doing so have no impact on this loss of trust.

Saudi Arabia is now trading oil in non-USD currencies for the first time ever, and the so called petro-dollar has been the anchor of the stability of the US dollar over the last 100 years. I put this in a separate paragraph because it is of huge importance, and deserves to be considered very carefully. Unfortunately, those in power in DC are obviously not doing so. Arrogance and ignorance are a lethal combination.

Argentina has relied on the US dollar for a very long time, but it has only to look at Cuba and Venezuela in order to see stark examples of what could happen, on a whim of some group of grandstanding US politicians. I would remind you that the current US ambassador to Argentina, in his confirmation hearings before Congress, described this country as "a pretty tourist bus with no wheels", and was then confirmed as ambassador, in spite of this horrifically contemptuous comment.
I hope that they are foolish enough to do so. It would be a great opportunity to short and quickly cover.
Idle chatter in the wake of the petroyuan. It would based upon what precisely? The full faith and credit of the Lula and Fernandez-Kirchner governments? An amusing thought and food for memes.
Even if by a miracle they could make it work, the next conservative government would just sabotage it, as they have tried very hard to sabotage Mercosur. (though, in fairness, even Cristina has at times seemed hostile to Mercosur)