I wholeheartedly agree with computers/systems, and keeping things separate there.. but two phones? Who wants to carry around two phones just for staying on top of slack during _off hours_?
If the company isn't ok with me using slack on my personal phone, then I'll only use slack on the supplied computer during business hours (eg. they get no mobile slack out of me at all). Either that or I find a different job. Life is too short to deal with so many devices and the hassle of it all.