I'm still trying to figure out double tounging myself. Growling, however, is something I can explain. Instead of just blowing an airstream into the sax, hum into it. It basically requires using more of your throat. Hum into the sax and just try to, for lack of a better word, roughen the the sound coming out of your throat. When I began to get this down, I couldn't go more than a minute or two without my throat getting sore. Once begin to get the hang of growling, adjust the intensity of your "humming" until you can growl without killing your throat competely. Also, growling tends to be easier to master on the upper half of the sax's normal range. Try practicing on the higher G initially; that seems to be easiest note to growl on. Once you master that, gradually work up and down from that note until the technique becomes natural.

i cant actually double tounge, but ill tell you what they tell me. first, go for a ti-gee syllable. the gee is when about 1/4 inch back from the tip f your tounge barely taps the top of your mouth. in order to remove fuzz from the sound and other sound killers, you can puff your cheeks or leak out of the side of your mouth. these are two big no-no's only way to get it clean.all of the best classical players double tounge like crazy. it will sound much more effortless than single when done right

Sax_mom and barisax999 have mentioned the best way to go about it, which is the traditional way. If you're able to do it that way, then go for it. It will take some practice, but it's the best way to go. There are some saxophonists who are unable to do it and resort to what is known as "doodle-tonguing" which is an up-down flicking of the tip of the reed with the tongue, similar to moving a guitar pick up and down on a guitar string. This takes significantly more control than the traditional method, and puts the tip of your reed at risk, but is a cool technique when done properly.