Pretty straightforward really.
After time T, each aircraft has gone through an arc of a circle of angle=(V*T/R) radians (assuming consistent units)
V=velocity
T=time
R=radius of turn
In cartesian coordinates, the X-distance travelled = R*Sin(angle)
and the Y distance = R*(1-Cos(angle))
The distance between the two is sqrt((x1-x2)^2+(y1-y2)^2) corrected for their initial position and separation.
About 5 minutes in Excel should give you the answer