Actors in multiple shows at once

Robert Reed had a supporting role as a police officer on Mannix at the same time he was on The Brady Bunch.

I'm pretty sure, Christopher Meloni was on "Oz" and "Law and Order: SVU" simultaneously.

I think Lisa Kudrow was on Friends and Mad About You.

The only example I can think of is Richard Anderson, who played Oscar Goldman in both The Six Million Dollar Man and in its spinoff, The Bionic Woman. Goldman was a prominent supporting character in both shows, appeared regularly in both, and they were both airing at the same time.

Now, I can think of quite a few examples of "cross-over" episodes where a character from one show does a guest appearance on another show. For example, Richard Mulligan played Harry Weston in Empty Nest, and also did a couple of guest appearances in The Golden Girls as the same character. But I don't think that's what you're looking for, is it?

Richard Anderson played Oscar Goldman on the Six Million Dollar Man and the Bionic Woman at the same time. He has the distinction of being the only actor to play the same role as a regular on two shows on two different networks at the same time.

David Hasselhoff in Baywatch and Baywatch Nights.

I believe Heather Locklear was on Dynasty and TJ Hooker at the same time. I will have to check.

edit: she was on TJ Hooker from 1982-86 and on Dynasty from 1981-89

Not acting, but Alex Trebek hosted Jeopardy! and Classic Concentration at the same time 1987 - 1991