Review the original item:
Ryan is highly allergic to cats,
he requires at least twenty god-bless-yous every time he visits Mariko
and her seven Siamese.
You chose the correct way to fix the error:
Ryan is highly allergic to cats,
so he requires at least twenty god-bless-yous every time he visits
Mariko and her seven Siamese.
Ryan is highly allergic to cats is a main clause. He requires at least twenty god-bless-yous is the second main clause.
You are always correct when you use a comma and a coordinating conjunction like so between the two main clauses that create the comma splice.