what can be detected by the casino is definitely the connection from the network used. but KYC, devices, and also the playing habits used must have differences.
many cases of using multiple accounts at the casino have been detected as committing possible fraud to increase the chances of profit obtained by gamblers.
if it has already happened like that, it is better for one of them to report to support and ask for one of the accounts to be closed. I think as long as the account is not indicated to have done something wrong in the game, it can be done. some casinos seem to offer this option to users who have multiple accounts on the casino's terms and conditions.