Can I access Shudder and Sundance Now on their own websites or apps?
No, you get all the same content, but not on the separate apps for those services. Instead, you will find all of that content* within the AMC+ app.
(Your AMC+ sign-in credentials will not work on the individual site and apps for Shudder, Sundance Now, and AMC. Individual Shudder or Sundance Now logins are also not supported on AMC+.)
*US and Canadian plans. Australia and New Zealand members, your plan has a different selection.