i make potholders all the time and i am having a bit of fog here trying to understand exactly what you are describing. do your potholders have a hanging loop? are you binding them without leaving a hanging loop and asking how to join the two binding strips together to enclose your squares?
i make my bias binding 2 inches, fold in half, iron, sew a 1/4" seam. i miter the corners. once sewn, i press the seam outward, fold over the potholder, add some glue stick to the binding, iron over the seam and stitch in the ditch.
if you want to join the two tails together i would say you need about a 3" tail left on each end. here is a tute excerpt that i am thinking of. i hope this is what you were asking.
I join the ends together with a diagonal seam:
I like to leave quite a large gap between the start and the end of the stitching, say 12 inches.
You will have two lengthy tails leftover.
Overlap the two tail ends smoothly along the quilt edge.
Trim back the binding tails so that they overlap by the width of the original binding strip plus 1/4 inch
In this case the tails overlap by 2.25 inches plus .25 - a total of 2.5 inches.
Be sure to use a ruler and mark the binding for the correct measurement.
To join the two ends with a diagonal seam unfold the binding.
Place the two ends right sides together, perpendicular to each other.
Allow the ends to overlap by a generous 1/8th of an inch as pictured . Secure strips with a pin.
Mark a diagonal line as shown.
Stitch along the marked line (which can be cumbersome with the entire quilt in tow)
You can now test to see if the binding fits along the quilt edge.
Trim back the excess fabric, leaving a quarter inch seam allowance.
Press open the seam.
Stitch the remaining binding onto the quilt as before.
This method of joining the binding ends works for me each and every time. It leaves a much less bulky seam, and is just like all the other joins in the binding.