If by Inner Shell you mean the inner isolation wall, or inner drywall assembly, assuming it's multiple layers and is a LF boundary, yes it's symmetry is important to me...
If by Outer shell you mean any heavy partition behind the above Inner shell, than I do not feel the symmetry is any where near as important. Its nice, but we never do any modal calculations on outer shell, so it they are not perfectly symmetrical because of other design considerations, than so be it.
IF by Inner shell you mean a framed system of acoustic treatments such as a stretched fabric or Slat wall, then yes the symmetry is important and this would imply that the "outer shell" was the drywall (or other) partition that is the first significant boundary to LF sound.... then YES the symmetry of this shell is equally important...
Wow.. does that make any sense??