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I should imagine it would involve blending two colours in the most litteral fashion- by bringing two strokes of paint togeather- it sounds like a horrible idea to me and far too much work though.
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From the title, If you mean two brush literally, then I remember there is a video on youtube on the subject. Problem is, I can't find it.
Brief synopsis of said technique:
Water down your paints a little bit (maybe 1:2 water:paint), then use one brush to paint a highlight line, then using a damp brush of similar or same size, and drag along the edge of the line towards the shade. It can look pretty good if done right, but it doesnt sound really that viable to me.
Hope this helps!
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