I understand why you want a co-owner instead of an employee. Been there, employees have excuses. Even when you see your company as a startup, you should treat the new project as a NEW startup. Ether spin of a new company with 50/50% for you and the co-founder or simulate that situation inside of your company. This is the same what you would do if your current company was a brick and mortar or if there would not be one at all.
This way your co-founder will have no problem to put in 120% of his time and energy (call me, may be ;-)) since your limited involvement (you still have to run your current company) will be balanced by the use of resources of your current company.