There is not currently, but its possible that MPAS Ocean could be coupled to MPAS Atmosphere using CESM, but there is currently no plan that I am aware of.
The differences in choice, MPAS-A in CESM and MPAS-O in E3SM and not in a single choice, comes from the differences in developers of each core. While the shared infasturcture of MPAS is shared between NCAR and DOE, the development of each core is not. NCAR develops MPAS-A and DOE developes MPAS-O. Likewise, CESM is developed by NCAR while E3SM is developed by DOE.
From my understanding (and someone may correct me), the reason for not coupling each core is simply because each organization (DOE and NCAR) have had different needs and objectives.