There are multiple options for attending MC as an MCPS HS student.
1. Middle College offered at some HS. I believe Northwood is one.
2. Ad hoc classes in the fall or spring. MCPS will pay for 2 of these classes per semester. Your kid could take evening, weekend, or online classes. Classes are still taken at the HS. If the schedule works, your kid could leave school early for a class.
3. Early College Program--this is the full blown AA degree program. MCPS pays for all classes and the program is 2 years. There are defined majors and they have required classes. You can't deviate from the requirements. Even the electives are relatively pre defined as they cohort the students. For this program, the kids no longer attend class at the local HS.
There is no defined advantage in obtaining a dual degree when it comes to UMD admissions. Even if they obtain their AA degree, they still apply as a freshman and go through the traditional HS college application process. Now, there would be an advantage if they were admitted in that the MC credits would transfer to UMD and your kid would enter with higher standing. But the MC-->UMD transfer option is not available for kids who do the dual degree program.