"Junior" and "Senior" are relative titles.
Semantically speaking, you would need minimally 2 PMs in order to quantify that one was "Sr" to the other. Comprehensively, you may need 3 or more PMs, since "jr" and "sr" titles define classes, and a population of 2 hardly creates differentiated 'classes'.
E.G. "junior" PMs would often report to the "senior" PM.... not just have a mentorship exchange with the Sr. PM. I hope you can see from the tiny illustration how such titles are really more about your company's organization structure, rather than having to do with an industry-absolute-skill level.
Why must you define "Jr" and "Sr"?
- Is it to motivate the employees? Are there retention and satisfaction issues that are being covered inadvertently?
- Is it to empower their career development beyond the scope of employment with your firm?
- Is it to define compensation levels?
Look to the driver for why such titles are needed. It may reveal a more important matter.