When I posted my Dark Angels power armoured troop the other day, I got an email asking about the actual spraying process I use to apply the split basecoat. It's not terribly difficult to do and I used it once before on my Ultramarine terminator shown above. After a little trail and error, I found a way that works really well for me.
The first thing you need to keep in mind is that your two colors need to be somewhat close in terms of value. I don't think this will work that well with white and black. You need to have a dark color and black. The value range between the two colors cannot be that far or the blending will not be very smooth. The further you are apart, the trickier it gets.
That being said, I've done this twice. Once with dark blue and black and a second time with green and black.
How you actually spray the model
Spraying the model is easy enough. I have two positions I spray for this effect. The first one is directly overhead. The second one is slightly off at an angle. Not very far though. You still want to be applying spray paint from an overhead position.
The same rules apply here as they do with all spray paint... shake well, don't do it in a hurricane, keep the distance between the can and model consistent and never start or stop spraying directly on the model.
With the first spray from directly overhead, I am looking to get complete coverage with my second color. I may make a few passes until I have no more black showing.
With my second spray at a slight angle, I will look at the model and only do this on the areas I want to add a little more color to. Looking at the model directly overhead, you can see that most of the model is covered. This second pass is to bring some of that color down onto the model.
I'm not going for complete coverage with my second spray. More like the suggestion of additional color here and there to break up some of the larger black areas that still remain on the model after the first spray. This second spray really is a matter of taste.
Things to keep in mind
Using this approach, I will wash my model with a similar colored wash to tie both colors together and help with the blending. You may want to go slightly heavier with your spray paint overall so that you can darken it down as you want. It's much easier to darken the model with washes than it is to try and bring that spray paint base color back up in value.
I use cheap spray paint. I look for the color more than anything else. If I can find the exact color I need, I go with it. And... it doesn't matter if it's gloss or not. Once you apply a few washes overall and varnish it in the end, you'll never know the difference. In fact, the Dark Angel I painted like this was done with gloss green spray paint.
If you're going to try this, do a test model or two until you're happy with the final results. Once you get a feel for how much spray paint you need to apply, you will fly through a unit of troops in no time at all.
Make sure to check out these posts as they might help:
How I paint Ultramarines fast and dark
How I paint Dark Angel power armour