How do I separate two Runs in the same TextBlock?

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C#

XAML

WPF

I have a TextBlock with two Run objects. I want one of the Runs to stay on the left-hand side of the TextBlock while the other Run stays on the right-hand side. I know I could try to insert a third Run between the two, but the number of characters in each Run changes along with the state of the program. An other issue is that the TextBlock is re-sizable, so I can't just subtract the combined number of characters in both Runs from some fix number and use the result to create white-space for a third Run.

Is there a way to right justify one Run while left justifying the other Run, or should I just use two seperate TextBlocks and disguise them as a single TextBlock?