Well, I think the way they look at that is that multiple independent triggers triggering different firing pins and different sears is not actually causing one firing system to fire multiple rounds, but two distinct firearms to discharge.
So like they wouldnt see the difference between that and firing two pistols in different hands simultaneously. Or if you duct taped them together and fired them that way.
Allow me to help.
A common take is that semiautomatic firearms are a privilege to have because they're not necessary for self defense. As a privilege, States have the right to regulate said semi automatic firearms. Including outlaw them.
The 1st ammendment reproduction here is
Documents of more than 800 words are a privilege to write and dessiminate because on average it takes less than 800 words to convey an argument or point. Therefore, as a privilege, a state has the right to regulate said level of speech since it exceeds the level of protected and becomes a privilege. A state therefore can outlaw forms of speech exceeding 800 words.
If that example doesng jive with you, another would be:
It takes on average 1m30s for a TV News agency to tell a story. TV News and their ability to tell stories is protected 1st ammendment speech, but, since it only takes 1m30s to tell a news story, anything on the news taking longer than 1m30s is a privilege and therefore can be regulated by the state. Including outlawed by the state.
A lot of people feel that regulation of the second ammendment is very scary because of the ramifications regulation like the ones proposed could have on other ammendments. Such as the like counterparts to regulating first ammendment speech I generated above.
As a real world example; I imagine if she could, Mayor Tiffany A. Henyard would see regulation of speech such as ive described above perfectly legal and in the best interest of her community in order to stop missinformation of her mayorship and the political agendas of The News in her area.
In a similar light, gun owners are seeing the regulation attempts of semi automatic firearms and are feeling very similar to how all of us would feel in the Henyard example above. For clarity, gun owners are feeling as though they are being told that the Government has the extreme authority to tell an individual citizen that has grown up with firearms, effectively and safely uses them, that said citizen doesn't truly understand what it is they have and that an individual collective of politicians ultimately knows whats best and safest for them... Many dont feel OK with that idea of giving up personal freedoms to some weirdo on TV that says "it has to be done for your own best interest". To those gun owners, it feels the same as Mayor Tiffany A. Henyard appearing on TV and saying "im regulating the local news agencies in the area based on average time to convey news that is not filled with political missinformation for the collective safety, progress, and betterment of our community and my ability to lead".