It is highly unlikely that a big legal dispute will not include both the personal accounts and business accounts from any witness (just look at the IRS investigations). However, from a POLICY perspective, you can set a clear policy that all work (and some personal, if your company allows) should be done through the business UC apps, and personal can reside in the native apps / other lists. You can then use your policy for regulations, some defensive positioning on discovery (at least you can make a big argument for compensation), and your own internal corporate needs (retaining records, contacts of customers, etc.). Is it a legal proof firewall - probably not. But it's way better than starting with everyting on the table.