The first sentence is easy to parse, as the two "that"s make it obvious what is on either side of the "and":
Jeff told me
that the job was still available
that the manager wanted to interview me.
The second sentence doesn't line up as well and could be interpreted in two ways.
The comma makes it seem less likely to have the same meaning as the first sentence. It seems more natural to parse it as:
Jeff told me that the job was still available,
the manager wanted to interview me.
In this case the information about the manager has nothing to do with Jeff.
It could mean "Jeff told me that the job was still available, and [later I found out that] the manager wanted to interview me.".
The second sentence is badly written and is ambiguous, but the comma perhaps help resolve the ambiguity, although that interpretation might not be what the author actually meant.