Not wise to ask buyers to send to each other. Consider the consequences of loss or damage, you would not be the sender so could not make any claim and buyers would not want to get involved.
They should both open Not as described cases, you supply return labels and on return you send out again correctly. This will prove expensive but at least younwill not be breaking ebay rules by giving out members addresses to other members. A complaint about this could give you a serious problem.