No, unfortunately, you cannot. In order to use multiple C.A.T.S. accounts simultaneously, you will need multiple devices.

Please DO NOT try to log in under different C.A.T.S. accounts on one device in order to play as different characters simultaneously. This will delete your initial character. You can only have one account on one device.

In order to properly use two accounts, you need to use two different C.A.T.S. accounts on two different devices. Otherwise, there may be problems with cloud synchronization and saving your in-game progress in case you lose access to your account for some or other reasons.