R. Shimon Ben Yochai said joining (שיתוף) is liable destruction. So rather it means they desired many gods.
How is R Shimon answering R Meir?
I think he is claiming "joining" is worse than regular idolatry.
At least that is how Rashi explains this saying that they in fact accepted other gods.
In any case, I think we can see clearly from this Gemara what the problem with the Golden Calf was. It was either adding something to god [that is R Meir's opinion] or it was worshiping another god in which case joining would have been worse.
Now once I was connected with Moharosh's group in Safed and I think they were giving hell to the local rav. [I am not sure of all the details but I think they had tried to take over a local building under building 7, to make it a Breslov shul. That is a law in Israel that once a building has been made into a synagogue you can't do anything with it after that.] In any case the Rav was bothered and so made a speech that was critical of Breslov. The idea of the speech was that the problem with the golden calf was not that they denied God but they said the God is everywhere and in everything and so it was the part of God in the Golden Calf that brought them out of Egypt. I felt he was being critical of me, but later I decided that I was mistaken in that notion. [Later he invited me back to the community after I had left so clearly he was not mad at me. ] Rather I think he was just being critical of that group. But was that in fact the problem with the Golden Calf? Not according to our Gemara here in Sanhedrin 63. Here in Sanhedrin 63, the Gemara is thinking God made the world and he is not the world. The trouble with the Golden Calf was adding something onto God. "Joining."
But what that rav said might be true anyway. It is just that you don't see it in our Gemara. They were not subtracting but rather they were adding.