Hello @MaxouGrosBidou. Apologies for the late reply.
Unfortunately, this is not something we can do. If you wish to play on silver with multiple accounts, you can register 3 additional accounts, 4 being the maximum accounts allowed per person.
What you could do is, create another account, trade all the items and Pokémon from your gold account to the new one, then transfer via the dashboard. This way, you can have all the items and Pokémon on Silver, but you would have to start again with the new account.