conside 0.1 and 0,9 - used to give 1, now would give

0.1 * (1-0.9) + 0.9
or
0.9 * (1-0.1) + 0.1

e.g.
0.91

why this instead of the old result, e.g. why shouldn't it be Math.min(1, this.alpha + other.alpha)

otherwise this looks good to me.