I have no experience of using 2 heads, but as a thought-exercise I would say that if you are planning to use a soluble filament for support, you will probably be better off putting support into your design rather than using the slicer generated support. Generated support must make a compromise between the incompatible goals of giving good support and being easy to break off. With soluble material the latter does not apply so you can make the support solid with no gaps between the support and the material it is supporting. With a completely solid support, overhangs should never curl up, and bridges will be as good as a base layer. Normal frangible supports do not prevent curl-up because they do not, by design, adhere strongly enough to hold the layer down, they are only good for preventing sagging.
One non-filament 3D printer I have looked at uses hot wax fired from multiple microscopic nozzles to print parts just like an inkjet printer, except it prints multiple layers. Just like an inkjet printer can print in more than one colour, this printer can print two different types of wax, one soluble and the other not. The designs for that printer are sliced to print a solid cube, with the wanted bits printed in the insoluble "colour" and the rest in soluble wax. The cube of mixed waxes is then put into a solvent bath and the wanted part emerges as the support material dissolves away. Thus the insoluble wax gives 100% support all around the part and any shape or print orientation is possible (providing it is self-supporting after being fully printed), with no consideration necessary for bridges and overhangs. The parts produced are used mainly for lost wax casting rather than being used as printed, especially for medical purposes such as replacement hip joints etc. where every patient needs a custom design.
Dave
