Maryland bars stacking ordinary UM, and enhanced underinsured coverage is the exception
Maryland's rules are in Insurance Article 19-509 (standard UM), 19-509.1 (enhanced underinsured coverage) and 19-513 (no recovery from more than one policy). The waiver of UM equal to your liability limits is in 19-510, the minimum limits in Transportation 17-103, and the deadline in Courts 5-101.
Read how stacked and standard coverage differ in Maryland, check what 19-513 and 19-509.1 say, or see how Maryland UM and enhanced underinsured coverage differ.