Shortening the number of reductions to prove NP-Completeness
Yes, that's possible.

The reduction from Subset Sum to Partition is a mapping $f$ from Subset-Sum-instances to Partition-instances.  The reduction from Partition to Job Scheduling is a mapping $g$ from Partition-instances to Job-Scheduling-instances.  So, the function $g \circ f$ is a mapping from Subset-Sum-instances to Partition instances.

This gives you a direct reduction in one step; the reduction is given by the function $g \circ f$.  If you work through the details of the construction of $f$ and $g$, you should be able to figure out what $g \circ f$ is doing.  If you wanted, you could write down the definition of the function $g \circ f$ without telling anyone that it came from composing $f$ and $g$.  Whether that would be useful in any way is a different question...