Hi Dudeness correctly points out that later models do not require the Gen to be turned off before second engine start, permitting a Generator Cross Start. I believe this only applies to aircraft with the 5 bus electrical system. However on the 90 I fly, this puts extremely high loads on the operating generator which I doubt does it any favours.
According to at least one guy (to whom I think His Dudeness referes in an earlier post... wrote a book, no?) on models with the older electrical system you can do Generator Assisted starts, whereby you turn off the Generator after charging the battery, get the second engine turning on the starter, then switch on the generator of the already running engine.
h