Disclaimer: I provide co-founder advice for a living [1], so take this with a grain of salt
I can see a couple of scenarios playing out here, here is the one that I think applies to you, but let us know if something else is going on:
That new person is mostly a sales person and you expect them to grow a fairly new product which is independent from your existing business. Fine. It's ok for them to have a huge incentive plan, where they earn most (half) of the profits of that new product. Compensation: decent base in cash, huge cut of sales, a small piece of equity (always vesting of course)