Just like that:
<input type="hidden" name="on0_<?php echo $ag_Store_Index+1; ?>" id="on0_<?php echo $ag_Store_Index+1; ?>" value="<?php echo $ag_Store->DisplayInfo("productSize") ?>" />
<input type="hidden" name="os0_<?php echo $ag_Store_Index+1; ?>" id="os0_<?php echo $ag_Store_Index+1; ?>" value="<?php echo $ag_Store->DisplayInfo("productSize") ?>" />
<input type="hidden" name="on1_<?php echo $ag_Store_Index+1; ?>" id="on1_<?php echo $ag_Store_Index+1; ?>" value="<?php echo $ag_Store->DisplayInfo("productColor") ?>" />
<input type="hidden" name="os1_<?php echo $ag_Store_Index+1; ?>" id="os1_<?php echo $ag_Store_Index+1; ?>" value="<?php echo $ag_Store->DisplayInfo("productColor") ?>"" />