Yes if you watch a 755 change over from diesel to electric you can watch the breakers close on the roof near the power unit (often with a spark and "crack" sound).Provided there's a 25 kV or lower voltage electrical connection between power cars, then only one pantograph isn't a problem Class 755 have a power connection between power cars in order to distribute the power from the diesel generators. Many types of train have two pantographs, but equally many types have only one. Pendolinos, for example, have two pantographs but only use one at a time. Conversely, Desiro and Electrostar 4-car units only have one pantograph, and if run in multiple have to have all the pantographs raised.
Single pantograph units have been operated for decades very reliably. In the particular case of the class 755s, if the pantograph were to fail, they would still be able to get out of the way on diesel power!
There's no high voltage connection between units working in multiple, AFAIK.