You would always need two string meters to get two colors. Period.
So now the problem becomes centering them together, and since there is no way to know the length of a string meter in pixels in Rainmeter, I don't think there is going to be any reliable way to do that, even with Lua. Fonts are pretty variable in how they are constructed and there is no certain way to use the name and size of a font and get exactly how many pixels a given text string is going to use. You are going to want to left or right justify this pair of strings, which is then trivial to get two colors with two meters.
Try using a Monospace font. Each character, including spaces, takes up the same amount of space.
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