Two Signals, One Ceiling

A researcher regresses a stock's daily return on signal A alone, with an intercept, and gets $R^2 = 0.16$. Regressing the same returns on signal B alone gives $R^2 = 0.25$. She now runs one regression on A and B together. What is the largest $R^2$ that regression could possibly produce, and what would the two signals have to look like for that to happen?