This is what the 2nd would look like if the first part was conditional:
As long as Sally agrees that, A well regulated militia, being necessary to the security of a free state, then and only then shall the right of the people to keep and bear arms, not be infringed.What it really has is:
A statement of fact: You may disagree that it is a fact, but it is intended to state something they thought was obvious:
"A well regulated militia, being necessary to the security of a free state,"
Followed by a rule:
"the right of the people to keep and bear arms, shall not be infringed."