No it isn't. That is the fundamental error most LNs and all CTs make. The second shooter is only required if JBC is reacting to his chest wound by z235 or so. But, if one follows the evidence, there was only one shot to that point. The last two were noticeably closer together to the vast majority of those who recalled a shot pattern. The second shot SBT is impossible with that shot pattern.
But that is not all. A second shot SBT is also excluded by the Connallys and about 22 other witnesses who said that JFK reacted to the first shot (in distinctly different way than the smiling and waving seen prior to z195 or so).
Again, that is not all: a second shot SBT is excluded by dozens of witnesses along Elm and in the motorcade who put the first shot after z186-z191.
A first shot SBT is excluded if you believe that the Connallys were right in their observation that JBC was not struck in the chest on the first shot.
So the demise of the SBT does not imply multiple shooters unless you are convinced that all these witnesses are wrong.
Three shots fired from the SN fits all the evidence. It just does not fit your subjective view that Gov. Connally must have been hit in the chest at the same time JFK emerges from behind the sign showing signs of his neck wound.
Describing JBC's chest wound as him being 'hit in the chest' implies a shot from the front, at least as far as I understand the written word. You are ignoring the back wound suffered by the guv, it seems to me.
Are you claiming a shot from the front indeed hit JBC?