I have a friend that's used a harness to walk her cat and the cat has excelled in finding new and unusual ways to get out of a harness during walks!
I think I'd worry about trying to retrieve a cat that slipped the harness while the other was still with me? I'm definitely inexperienced with cats, though.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."