In addition to Mechman's solution.
Don't extrude/cut the basepart, but turn the part
into an iPart.
Use the Mirrorplane as Splitplane for both parts.
size=2>Like: Split1=Basepart
Split2=Mirrored
part
In the iPart author you can suppress Split1,
Split2 or both.
You 're also be able to suppress features on
the mirrored part, and add features to it.
Have fun,
Henjo
P.S.,
If you need an example of this let me now.
(1,5 Mb)
Enjoy The Intelligence ©
J.Jimenez