Well, HH Diamond can be had for zero stays - you just have to use their credit card and live with $40,000 worth of CC spending accumulating somewhat less reward value than you'd get from the SPG Amex.

Marriott Platinum will typically need somewhere in the mid-50's nights-in-beds assuming you don't make the MR Visa your primary spend CC everywhere else. (Depends on your total Marriott spend on the Visa.)

So keeping them both isn't too hard if that's your goal. If 28 stays represents 3 months, you'd be MR Plat in six.

That said, the real benefit of Platinum status is at the full-service brands. Platinum at Fairfield Inn doesn't do you much good. Hampton Inns are generally better than FFI's. If that was my stay pattern, I might be tempted to make Priority Club my backup plan. They tend to show up in tiny towns where even FFI doesn't appear. (The main brand where you'd redeem PC is Intercontinental Hotels. There aren't thousands of those, but they seem to have *decent* global coverage.)