Few ways to approach spraying fungicide:
1. Split tips.
Since it is fungicide, you don't have to worry about drift so much as you would with herbicides. But, drift is still wasted chemical, so you want to get something inbetween drifting (waste) and getting great coverage.
There's a few options in the market (Hypro split-cap, Wilger y-Adapter, Teejet Twin Turbo), and some of those allow for drift reduction caps to be used. From what I've seen from testing, it seems having a shallower angled tip works better, and the tip pointed forwards does the majority of the work. With that in mind, I'd recommend splitting the volume up somewhere between 50/50 or 70% front/ 30% Rear for the tips. Also, if you are using conventional flat fan tips (not drift reduction), keep in mind that is going to mean you are getting a lot more drift than if it was coming from one tip.
If you have AIM, you wouldn't be able to use air induction tips, so your options would probably be Wilger drift reduction tips (same results as AI) or using conventional flat fan and dealing with a lot more drift.
2. Single Tip - Coarser Tip. Higher Water volume.
Since you need coverage everywhere and you are using a coarser tip (or even a regular tip at higher volumes will produce coarser droplets than recommended for fungicide), you will need to compensate and use more water volume than you might have otherwise. (Sometimes, this might mean sticking to the label rate, rather than cutting water down like most guys might)
There's a few ways to optimize this setup as well to make it work pretty darn close to using a dual tip setup.
First is adjusting the spray angle. From testing, it seemed like even using a single tip rotated forward (~30°) will optmize your coverage for most fungicide applications (i.e. head blight, hitting the flag leaf, etc.). Essentially what happens is the sprayer speed (10+MPH) causes the spray pattern to pretty much travel horizontal. Provided spray height is consistent, and at the right height, having that spray going horizontal will make it a lot easier to hit your target. This might be a little different for rust suppression, as you need pretty decent canopy penetration, but I'm sure you can still cater to your application.
As far as an anecdote for this, think about trying to karate chop a head of wheat in the field, while travelling 10MPH+. Now, try swat at it with a fly swatter.
Which is easier to hit the head?
In Saskatchewan at least (I know some testing had been done in the US as well), Tom Wolfe and PAMI (Prairie Agricultural Machine Institute) are doing testing for Wilger (and a handful of other major tip manufacturers) to determine what the best way of fungicide spraying would be. From my understanding, this information will become public when it is all said an done as well, so we will wait and see. The first set of 'in-lab' testing has already been done, and the first set of 'in-crop' testing has been done and data is being accumulated. The intent is to do another round of in-lab and in-crop testing next year as well. (Maybe a third year if needed as well)