It's unbelievable 20 posts about this.

Equation a = k*M/r^3 is frequently used in astronomy. There is nothing wrong with it, look at Wolfram pages, he mentions it as well.

Actually it could be also writen in form k/(va DOT va)^(3/2), or in the form
v = v12*k/d^3
a1o = a12 + Mass1 * v
a2o = a21 - Mass2 * v //Apply from smallest Mass*v to largest