Take it BACK to whom ever did the head job ASAP!! Did they pressure check the head and then resurface it? If not, it may be cracked or warped. You MUST resurface a MB head each time you remove it to do a head gasket replacement.
I would guess that you don't actually have either spark or fuel on those two cylinders. If you have compression, spark and fuel injected into the cylinder, it should run. Check for spark, check for injector pulse on the suspect cylinders. Check compression too. If you want to find a MBZ specialist, here's a directory link for you: https://repairpal.com/directory?address=93309&car_brand_names=Mercedes-Benz