Making Qty Discount work with Products with Attribute Stock mod.

I'm currently facing an issue with the proper set up of the checkbox attribute for a gift product. The gift comes with 3 different color skirting, namely white, red, and blue. I have created each color as an attribute and have subsequently tied them to the gift product.

I am using the Products with Attribute Stock mod. and had set up the quantity for each color accordingly.

I want to allow customers to select any/all checkbox and offer quantity discount if 2 or more t-shirts are ordered regardless of the color chosen. However, when customer ticks more than 1 checkbox, the following warning is displayed at the shopping cart page:

Products marked with *** are out of stock or there are not enough in stock to fill your order. Please change the quantity of products marked with (***). Thank you

How can I set up the above-mentioned pricing system? TIA.