User A - can't be part of sudo or admin group, ssh keys based identity setup for ssh login, can't be a guest account
User B - no home folder, must have a password, can't have independent ssh login, should be part of sudo / admin group
User A don't need any sudo access for most of it's operation. But for some emergency cases, when root access is required we have to switch to User B while logged in as User A.
While switching from User A to User B, we should use User B's password. I.e. only some people logging as User A should be able to switch to become User B.
Please help in setting up such users. Thanks in advance.