I believe this has been discussed before. The issue is the C20 series engine and the fragility of the generator shaft which has a history of shearing under excessive load.
Later A109E and others have different engines.
The AS355 is similar to the A109AII and 109C but the manufacturer has slightly different procedures. Generally you should not cross start a C20 and should use ground power or the battery, in order to prevent damage of the generator shaft of the running engine.