Determination I

Given three data sets $X_1, X_2,$ and $Y$ , we run two linear regressions to obtain $y \sim \alpha_1 + \beta_1 x_1$ and $y \sim \alpha_2 + \beta_2 x_2$. The $R^2$ value for both regressions is $0.05$. Find the greatest lower bound on $R^2$ value of the regression $y \sim \alpha + \beta' x_1 + \beta'' x_2$.