A couple of ideas for you,
jimc11, that come to mind quickly. Ideally, I think you want a single ownership and while it's not impossible to find one in area you seek with 2 weeks attached, it may be a bit difficult, so perhaps 2 separate timeshare ownerships may work for you. Better than that, a multi-destination, points-based timeshare ownership with enough annual points to acquire 2 weeks in a 2BR may be your best bet.
Wyndham Vacation Resorts &
Bluegreen Vacation Club are 2 multi-destination, points-based ownerships that have resorts in Myrtle Beach.
Hope this helps!