Y ~ X1
Y ~ X2

df <- data.frame(
  x1 = c(0, 0, 0, 1, 1, 1),
  x2 = c(0, 1, 2, -1, 0, 1),
  y = c(0, 2, 4, 2, 4, 6)
)

> summary(lm(y ~ x1, data=df))$r.squared
[1] 0.2727273
> summary(lm(y ~ x2, data=df))$r.squared
[1] 0.2066116
> summary(lm(y ~ x1 + x2, data=df))$r.squared
[1] 1