I'm a SixBit user, you could sign up for a free trial, add both selling id's.
You can then download from one account and then upload under the other account.
I presume we are talking about a large number of listing, in that case you might find the monthly subscription is well worth the cost. It's worth it to me (US$42/month plan) even though I have a much lower volume in recent years than when I originally started using SB (actually predecessors) 21 years ago. Still it prevents expensive mistakes and saves a LOT of time.
https://www.sixbitsoftware.com/clients/aff.php?aff=242

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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