Blending is almost an art form John, mixed results are very common. I remember the first clear blend I did on a C pillar that I didn't need to polish...I thought I had it mastered...until the next one I did...LOL.
I've had the benefit of working with a number of shops through out my career and unlike working in one shop (which does have benefits as well) I worked with numerous shops and was able to learn from many technicians...and very quickly you learn the good techniques from the bad...there's plenty of them as well.
One shop I dealt with in particular, all they did was spot repairs...and did quite well doing it. If you had a ding in your bumper cover, that would cost about $250, (a body shop would charge between $500 and $600 to repair and paint a cover flat rate)...if you had another ding in your fender and it was the same color, that would cost about $150, they averaged 8 to 10 cars a day, so the painter needed to be damn good at blending clear's and single stage. They used every technique, every piece of equipment that would speed up production like UV Primer that cured in a matter of minutes, fast clears with infrared heat lamps every where...it was quite a sight to see and I will say, I did learn a lot from that painter and the other people working in that shop as well.
OKay, when you use the two gun method the prep is the same, again, reverse masking (I hope I explained that well enough in my previous post) is very important. As with one gun, apply paint over the repaired, primed area (just as before, if it's a scratch that your repairing, dial your gun in so that your fan, volume and air pressure are sufficient to cover the primed area). Once you have coverage with allowing proper flash times between coats, remove 50% of the paint in your gun, replace it with blending solvent and purge the gun. With the other gun, put straight blending solvent into the pot and mist a coat of blending solvent over the prepped area, with the other gun, mist a coat of 50% catalyzed paint/50% blending solvent several inches past the area that you have just achieved coverage...allow some flash time, just enough so that the blending solvent has started to evaporate and you can visually see your prepped edge. Again, mist blending solvent over the entire prepped edge. Leave the 50/50 solution in gun #1 and immediately mist on another coat of the 50/50 solution stepping it just a little further than the last pass, even less flash, repeat a mist coat of blending solvent followed by an immediate pass with the 50/50 solution. The reason I don't add more blending solvent to the material in gun #1 is that I have the extra blending solvent already sitting on my prepped area that I haven't allowed to totally flash. There are instances where I would add more blending solvent to my paint, even with the two gun method, those would be colors with a high metallic content or if I'm blending larger areas, blending solvent flashes a lot slower than any reducer and that way on a large blend I can keep a wet edge). Repeat until the prepped area has been covered (or very close to completely covered...the over spray should melt into the remaining small area). Allow it to cure, hand polish if needed (chances are on Single Stage you will need to hand polish) and the repair should be invisible. I hope I have explained this well enough, if you or anyone else has any questions, just ask and I'll try and be more clear.
One reason I like the two gun method better for Single Stage and clear blends over a large area is that you have more control of the amount of blending solvent, if you have a dry edge you can mist on blending solvent and the dry edge goes away and for the fact that Acrylic Urethane's are a Bear to blend, with pure blending solvent being misted on the prepped area, it makes the substrate more conducent to accepting the freshly applied paint and less chance off peeling back.
The painter in the shop I mentioned earlier would average polishing 3% of his jobs (yes, the shop kept track) and about 8% to 10% of the jobs they did where Single Stage. This is what the painter did all day every day and when it came to blending color and clear, I haven't ever seen anyone better. I didn't sell them a lot of toners, a fair amount of clear...but they sure bought more UV primer than any shop I ever had before.