Why not simply maintain 2 separate tables and union them when needed? What value do you get out of jamming two separate concepts into the same structure? For that matter - why not simply keep the distinction between acocunt and contact separate from the incremented column? What happens when I need to delete account 1234 and I already have 25000 accounts in (i.e. the sequence will now be "broken").

If contacts and accounts are that similar in nature - what happens when a contact needs to "become" a contact? is there any case when they switch back and forth?