What distinguishes the case of transferring a debt of one thousand to two co-debtors (allowing collection from either) from transferring a debt of two thousand to one person?

Primary text

The distinction lies in the fact that in the case of the one thousand debt transferred to two, there is no quantitative advantage, as collecting the one thousand from one settles the entire debt. Conversely, if the debt were two thousand transferred to one, collecting from one leaves a remaining balance on the other, and there is a real quantitative advantage (tafadul) in the latter scenario.