Has anyone tried running two different size nozzles on a dual head?
For example, putting a “course” 0.8mm nozzle on one head while running a “fine” 0.35mm nozzle on the other head.
The 0.8mm would lay down the bulk of the print, (like infills, and thick sections, etc.,) while the 0.35mm would just do the fine details.
Has anyone tried this?
In theory, it seems like it would offer the benefit of fine detail without paying the penalty of greatly increased print time. You might even figure out a way to tell the slicer to run the fine nozzle layers in some multiple (2x or 3x) of the course nozzle layer. Could possibly speed up 3D print time as much as 10 fold if you optimized the process. You could certainly increase the print quality a great deal.
Just a thought…
Bill D.
I don’t think this would improve your print speed much against a pure small nozzle print. What takes time is always the perimeter, the infill has no big impact on print speed because you can crank up the speed for infill nearly until the ability of melting filament of your hot end is reached.
If your print speed is limited due to an inadequate x and / or y axis this is a different thing, but then it might me much easier to fix this instead of playing with different nozzle sizes which gives you a lot of new problems like oozing of unused nozzle etc…
I think the slicing program uses the same layer height for both size nozzles so it might not work to well.
Depends on the slicing program. In Slic3r, you can set the infill height at multiples of your perimeter layer height. And you can choose another extruder for infill…
HMM! I must have a bad version of slic3r(1.2.9) then, mine does not let you set a infill height. 
I can change width but not height in the version I have.
Maybe you look at the wrong place?
If it’s not there, it’s not in 1.2.9. But this version is quite buggy anyway, I would recommend to compile the latest dev version…
The new flexydually uses this configuration with a .5 for T0 (normal extruder) and .6 for T1 (flexystruder). Its primarily for optimal extrusion of the softer filaments.
You could setup a dual prints that use the finer tool for perimeter and the coarser tool for infill… that could improve detail and possibly save some time. The layer heights should be consistent otherwise you run the risk of the nozzle running into the print.
In S3D they have a parameter called “Print Sparse Infill Every” which I believe is probably the same as the “Combine Infill” parameter in Slic3r. The parameter specifies the number of layers to skip before printing an infill. For instance 2 would mean an infill printed every other layer… 3 should mean every third layer to print an infill. I’ve never used it… but its purpose is to “speed” up prints.
So you say s3d can’t handle different extrusion widths for different kinds of things like perimeter, infill, …??
That is how many layers to do something at, and has nothing to do with different nozzle layer heights. a .8 MM nozzle is 2.28 layer heights for a .35 MM nozzle. Plus that is only a setting for infill, and if you want the model layers to be different in vertical sections is a totally different conditions than infill.
If you change that value it is not doing infill only at every x layer, it is in fact a thicker layer height for infill every x layer. 
Support material is also printed at variable (most times higher) layer height than the rest of the part. You can also set the layer height seperately for different part height levels and change the extruder with some extra gcode at this events. Maybe I’m not understanding what’s your goal, but I can’t think of a situation that’s not possible in Slic3r…