Two separate accounts or one — how do I choose?

One account if you're the boss of both and want everything in one view, with the same login and possibly one bill. Two accounts if the structures differ: different partners, separate bookkeeping, or a possible sale of one. The honest point that really matters: data doesn't split with one click after the fact — if the two businesses could ever be sold separately, start on two accounts from day one. The price stays the same; only the view changes.