Neurologist can help
A small amount of information is on the internet regarding MAO inhibitors (such as selegiline) and l-dopa to treat parkinson's.
Among elderly patients, individuals can react quite differently to various meds & doses, so a person's treatment has to be customized. It appears that using both meds allows one to use a lower dose of l-dopa. The list of selegiline side effects looks milder than l-dopa's.