When you create an email in Preferences> Communication Builder, there is a field for Signature (From/Reply-to user). This selection will be overriden by whatever campaign contains that email message.  So you can have the same email in different campaigns, each with different From/Reply-to user/email address.
How to set Signature for an email in Preferences> Campaign Builder.
NOTE: your campaign must be in Pending status when you create or change this setting.
  1. Click your campaign name to open it. If your campaign is active, click the Deactivate button, refresh your screen and click the campaign name again.
  2. Click Next Step until you are on Step 3 Choose Messages.
  3. Click and drag the Email button into the big grey space. A pop-up appears for you to select the email you want from Communication Builder.
  4. Send to Assigned Rep defaults to Yes; if you would rather everyone get the same Reply-to address click No and select the appropriate user.
  5. Save to close the pop-up window and repeat #3-5 for any other emails you add to this campaign.
  6. Save the screen to go to Step 4 Preview & Activate.
  7. When you are ready, click the Activate button on the left.