That depends on your implementation. Both Date (timestamp) and Marketing Cloud Visitor ID information are available in all of your Omniture calls. If you're capturing these values in eVars, you can create a segment based on Visitor (as opposed to Visit or Hit, which won't work) to look for the two specific dates you want. Then, apply that segment to a report containing the user IDs. If you have a different variable containing custom user Ids, this should work as well.
Let me know if you need any additional clarification.
Also, if Workspace has disappeared, you may need to ask your company's Adobe Analytics administrator to ensure the user group associated to your profile has a checkmark next to Analysis Workspace Access and Create/Curate Projects in Analysis Workspace on the Analytics Tools portion of the group management page.