Help! I need to Pass Multiple Names & Values with One Radio Button Form

[Jimstershat]

Hello Everyone,
I need to do the following;
I need to create the following url using a radio form:

http://www.domain.com/addtocart.aspx?productid=18&variantid=37&quantity= 1

I need to be able to let the customer choose different product ids that always have a variant id associated with each product id. In other words If customer chooses product 1 it will always have a variantid= 10 and product 2 will always have variantid=20. So .. I need to pass a product id and a variant id associated with one radio button. Can it be done??

I have the form working below but it only allows for one product choice because the variant also changes with each product but the customer does not need to select the variant.

<FORM name="RadioGroup1" METHOD="post" ACTION="http://www.domain.com/addtocart.aspx?">
<input type="radio" name="productid" value="18">
<input type="hidden" NAME="variantid" VALUE="37">

<INPUT TYPE="hidden" NAME="quantity" VALUE="1">
<INPUT TYPE="submit" VALUE="Add To Cart">
</FORM>

My idea was to have
<input type="radio" name="productid" value="18&variantid=37">

but that didnt work

Thanks in advance to someone who knows much more than I do :)
Jimster