The constraint of no more than 2 layers is based upon asphalt singles and the total load on the roof. Regulations vary by state and town. The added cost to remove the shingles and dispose of them shouldn't be too much in the overall scheme of things. Take the opportunity to examine the decking of the roof and fix any problems now. finding them after the metal installation will incur a much higher cost to repair (possibly more than the cost to remove the shingles now).

Also, with only the metal roof, the load capability of the roof will be higher for things such as snow (if you get snow).

Overall, I would remove any existing roofing prior to installation of the metal roofing material. You will get a better roof.

Dan