I would say "This contract will take effect after being signed and sealed by both parties." Like kraut9 I do wonder what you mean by "sealed".

Thank you guyes, it is very helpful!

I think the second one sounds more correct but the word ''sealing'' should be replaced by ''sealed''

Hi it is the second one you will want to use but using the word sealed would be even better Formal documents are written in the past tense so sealed is the professional wording here. You could also put "the contract will take effect once it has been signed, sealed and witnessed by both parties" Most contracts needs will need a witness. Hope this helps

The first one sounds more clear to me. "Sealing" for me means the closing of an envelope, or closing something. But not to place a seal on something. I might be wrong though.