Not an expert in SoGo but I believe this is per user setting in SoGo so you can not change it on identity level. https://www.sogo.nu/files/docs/SOGoInstallationGuide.html
U - SOGoSentFolderName
Parameter used to set the IMAP folder name used to store sent messages.
Defaults to Sent when unset.
Use a / as a hierarchy separator if referring to an IMAP subfolder. For example: INBOX/Sent. You must use a / even if your real IMAP separator is a ..
I would guess you can make a workaround by SOGoMailAuxiliaryUserAccountsEnabled
D -SOGoMailAuxiliaryUserAccountsEnabled
Parameter used to activate the auxiliary IMAP accounts in SOGo. When set to YES, users can add other IMAP accounts that will be visible from the SOGo Webmail interface.
Defaults to NO when unset.
This way you could use 2 (or more) IMAP accounts in one SoGo session that will have separate Sent folder (if it helps you).