To list in two categories, two different categories must be specified

Usually, eBay returns this error when the exact same category value is set for Primary and Secondary Categories at the same time. 

To edit eBay Category settings in M2E Pro:

1. Go to eBay > Configuration > Categories and click View next to the category that the affected products are listed in.

2. Next, click on the Primary or Secondary tab (depending on which one you wish to change).

3. Select all of the products that you want to update and opt for Edit Action.

4. In the pop-up window, you may search and apply a new category.

5. Please remember to Save all of the updates you make

Alternatively, you may:

1. Go to the Listing, and switch the view mode to Settings.
2. Select Edit Category Settings in the Actions drop-down for the affected products.

Having an Item listed in two categories will bring it up in searches and eventually increases its visibility. However, not all of eBay categories allow listing in a second category.