I am NOT sure what EXACT issue one might face if they run LYNC and SFB on the same MAC. But our Documentation states that running LYNC and SFB on Same MAC is not supported
Side-by-side installation of Skype for Business and Lync is not supported on Mac. We recommend that you perform a clean uninstall of Lync for Mac 2011 prior to installing Skype for Business."
I have seen cases where Clients crash often in such a scenario.