I thought that the idea was to introduce pantograph vehicles, which will supply electrical power to the existing five Onix traction sets on the existing five motor cars. The traction equipment should be able to have additional control equipment introduced, so that changeover from diesel to electric when the pan is raised happens relatively smoothly.
I'm puzzled as to why two pantograph vehicles are needed (I think I've read that?). It could be that it's to strengthen the fleet, but in all but the six x seven car units, the maximum power demand should be no more than 3 MW (including auxiliaries), which one panto car should deal with comfortably (Pendolini can draw nearly 5 MW through one pantograph and main transformer).
As for power to weight ratios, if unmotored panto cars are introduced, I reckon they will weigh about 45 tonnes (challenging, but possible). This will increase the weight of a five (now six) car unit to 289 tonnes, with 2750 kW of available power, so a little over 9.5 kW / tonne... more than enough power to cope with the MML.
As a comparison, an HST (2 + 8) weighs 414 tonnes and has c. 3000 kW of traction power, so a power to weight ratio of 7.25 kW / tonne.
If preferred on the 222s, one engined, motored vehicle could be removed from each of the existing seven cars and added to some of the five cars, to make a larger, more uniform fleet of seven cars (once the pan car is added). That would mean a revised 222 fleet of twelve x 7-car (222001 to 12), eleven x 6-car (222013 to 23) and four x 5-car (222101 to 104).