Honestly, I wouldn't do it on the same device. There have been numerous reported instances recently of people who access multiple accounts on the same device being at least shadow-banned for a period of time. Best bet would be to use a second-phone (or her phone) if you want to work her account simultaneously.
I had a catastrophic failure on my Android phone last night. Got a new phone this morning. Downloaded the PokemonGO app and it logs in to my account without issue. I also work my wife's account because she broke her ankle and still has trouble getting around much. I'm trying to figure out how to add her account on my phone so that I can log out of my account and log into hers. Any helpful suggestions?
My problem is that it no longer gives me the option to log into my wife's account. When I log out of mine, it goes to the Pokemon GO log in screen. I sign in as "returning player." This takes me to the screen with Facebook, Google and PTC. I tap Google. In the past, it would take me to a screen that had my email address and her email address, and I could select which account to open. Now, it simply logs into the game for my account. It doesn't give the option to select her account.
My wife broke her ankle a year ago and still has trouble getting around. She can play while I'm driving or for short walks, but if she's on her feet for an hour her leg swells up past the knee. It's insane that it is taking this long for her to get better. I walk her account for a while every day to keep streaks alive and to occasionally raid or catch interesting/high iv stuff for her if I spot it while on my account. Seems to me it would be a bit harsh for Niantic to institute a shadow ban under those circumstances. Caught a 2076 Latios for her yesterday.