Hi,
Each Auctiva account/ID can operate with only a single eBay account/ID and vice versa.
You might file a Support Case and ask if there is any way to consolidate/reduce the billing amount based on combined use of the two accounts. They made a few early remarks (in posts) of that possibility, but the very low flat-rates now might make that situation moot. It never hurts to ask...
Danno