The fusion becomes even more unstable than anime Merged Zamasu, as the fused body gets overwhelmed by so much maximum power and muscle mass, that they defuse, even quicker than current Vegito.
... But assuming fusion time limit isn't a factor, then probably stops at Jiren, or somewhere before.
If we include what transpired with God Broly, this being likely clears with ease. God Broly was the only villain ever in the history of DB/Z/S to have to be defeated by breaking the 4th wall and fusing with everyone outside, creating a being with no name, no longer being Goku or the Audience, so let us call him Godience. This fusion in theory even trumps the 5-way Ultra Fusion from Dragon Ball Fusions, as it consists of numerous beings from outside of the realm of DB.
He even fused with me I remember him reaching out and I reached back. He fused with you too, you were a part of Goku....never forget that when life gets you down, you helped prevent the end of everything, whether you were aware of it or not. Each of us holds an untold power that if we were transported to the DB realm we truly would be Godly to an unfathomable level...Think Mxy when he drops to a lower dimension.
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Ahem, but if none of that is added into the equation since it's too much maximum power, then I figure they stop at Jiren or somewhere before.