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Pair Trading - Negative Beta

On the Pair Trading Lecture we learn that:

Z = X2 - B*X1

When Z < 0 (buy the spread), Z > 0 (sell the spread).

In case Z = 2, I would SELL ONE share of X2 and BUY "B" shares of X1.

But what happens when the calculated beta is NEGATIVE?

I would buy both?