you don't say what kind of batting you have...if it is polyester it (does not breath) tends to be warmer- cotton batts breath- and are often a lower loft-
you can double batts- many quilters use a cotton & a wool, or 2 poly's - or some other combination to give more definition to their quilting stitches. as for warmth- a flannel back would add some nice warmth to your quilt
depending on how you plan to quilt your quilt doubling 2 medium loft batts could make it a bit difficult to quilt...but again-it kind of all depends on what kind of batting you are talking about.