[Suicide Junkie]

(Position difference) = (speed diff) * T + (Accel Diff) * T * T

After time T, you want the position change due to Speed and Accel to be equal to the current position difference.

I think I missed a negative on the left side:
X1=0; X2=4
V1=0; V2=0
A1=2; A2=0
-(0-4) = 0-0 *T + (2 - 0) * T^2
4 = 2 T^2
T = sqrt(2)