I use 6-3-16 fertilizer made specifically for bananas. I buy it from AFEC(short for Atlantic Fertilizer something). I've had great success so far. For those of you in South Florida, it is on 260st and 182nd ave more or less. You can get more info from their web site. I think they do mail order as well. Oh, and the fertilizer is 50% organic...better than nothing right?
As for doubling fertilizer for desired effect...it doesn't work that way. If you want 0-0-30, you need to buy 0-0-30. Feeding double the amount of 0-0-15 will only be adding way too much fertilizer. The only time I've mixed is when I apply blood meal(12-0-0 I think) and bone meal(0-6-9) to my veggies. I'd like to use them for my trees as well, but going organic can cost a pretty penny.