If you put the ceramic coating first, the XPel won't adhere to it, so if you want both XPel and Ceramic Coating, the XPel goes on first.
Also, the XPel healing is powered by UV light, and that passes through the ceramic glass coating, so you still have the healing feature. The ceramic coating is hydrophobic and helps repel water (which can leave mineral marks) and helps keep the vehicle clean. It's much easier to keep a ceramic coated vehicle clean, not to mention it give it a more glossy finish.
Also, the XPel healing is powered by UV light, and that passes through the ceramic glass coating, so you still have the healing feature. The ceramic coating is hydrophobic and helps repel water (which can leave mineral marks) and helps keep the vehicle clean. It's much easier to keep a ceramic coated vehicle clean, not to mention it give it a more glossy finish.