No, unfortunately, you cannot. In order to use multiple C.A.T.S. accounts at the same time, you will need multiple devices.
Please DO NOT try to log in with different C.A.T.S. accounts on the same device in order to play as different characters at the same time. This will delete your original 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 game progress if you lose access to your account for any reason.