For software development, I feel like being "overemployed" would be relatively easy as a mid-level or senior developer. Job #1 is your 'mainstay' job with decent pay/benefits (but requires more concentration), Job #2 would be the 'side' job that you could fulfill your responsibilities with minimal effort.
A full-stack dev with 7ish years of experience could easily create/maintain CRUD apps for a smaller company with minimal effort while focusing the rest of his efforts on a more challenging role at a larger company.