draw seal out of assembly

[popo]

Hello

I have two parts linked with bolts, containing oil (it is a gear reducer)
so the seal will have the shape of the common area between these two parts
like this seal for example http://ills.bmwfans.info/ar66.png

how to get it automatically? if possible with update if I change the parts

thank you

 

[MrCATIA]

COPY the shape from the first part, and PASTE SPECIAL WITH LINK to the second part as well as the seal part.