You asked, “Can public officials serve more than one master at a time?”
My interpretation of both the city and county commissions is that the commissioners serve the voters, not another master. If a candidate meets the requirements for both positions, they should be allowed to run.
From my perspective, I would rather have a person on both commissions than have two former school board members on a five-member commission. I have watched school boards across the country for 50 years and most members become convinced that adding more money is the solution to all problems and tend to forget they are tasked with conserving tax dollars. Too many school board members could approve additional funds, which will increase the tax rate.
Should David Tomita win the city seat and decide to run for re-election to the county seat in 2014, the voters will correct that if he doesn’t perform as expected.
With a seat on both commissions, Tomita would be privy to data not available to all citizens and could help the residents of Johnson City get a fair shake in county decisions.
To answer the original question: It is legal, he wants to commit the effort to do both, he would appear to have the credentials and the voters should be allowed to decide if he should serve on both commissions.