showing available product quanity in two different units in openerp / odoo

i am working on odoo to implement for a liquor trading firm. Actually clent uses two different units e.g., unit(s) and case. Case size may vary. for example 1 Case = 12 or 18 or 24. In odoo if the default UOM is case then it shows available product quantity in decimal. In any time if the available product quanity is 15 (for case size 12) how can it be possible to show the quantity as 1 case 3 unit(s) or 1.3 instead of showing as 1.25 case. please help out of this situation.

thanks