Unless you're working 80 hours a week you're not doing two jobs. You're doing one job.
They've already gotten away with adding infrastructure and architecture (aka system design) rolled into one developer position. And putting it behind long and stressful interview processes. I'm not doing PM stuff on top of all that and not getting the pay and prestige for it.
Perpetually doing less in fear of not getting paid enough for doing more is how you get paid a pittance while complaining about it constantly. Doing more and then finding a way to get paid more is how you get paid more and be happy.