I tend to think that shared commissions will lead to friction no matter how you slice them, so your best option may be to divide the shop based on your product line.
Basically split the responsibilities and commissions into departments. Given that your shop sells clothing and shoes it may be best to make "employee A" responsible for selling clothing and "employee B" responsible for selling shoes and rotate them as needed.
This will hopefully reduce the direct competition between the two employees in any given day and offer clear lines on who gets what commission.
So, "employee A" may sell the customer a dress and get the commission on the dress alone and then send the customer to "employee B" theto get the matching shoes and "employee B" will get the commission on the shoes alone.