Brass is 60+% copper. Copper to brass is an extremely friendly combination to solder. I don't get the part about heating the copper first. You should put the heat mostly on the brass. By the time it is hot, the copper will also be plenty hot.
People have soldered brass over the years with propane. Plumbers often use acetylene. And now, MAPP is available . Some torches will use both. If you have the basic propane torch from the hardware store, the orifice and tip in that are probably not right for MAPP; but a new kit won't set you back too much.
The reason a hotter gas is better, is that time is your enemy here. You want to QUICKLY bring the metals up to temp., apply the solder, and DONE. If you have to keep the smaller, cooler propane flame on the jont for a long time to get to temp. this is when the flux burns off, and also the metal can become "unfriendly".