Perhaps this will help clarify...
Ownship is moving on heading 30° at 4kts
At 10:15 radar reports contact M1 at 36.0k yds bearing 270°R
At 11:00 radar reports contact M2 at 36.5k yds bearing 260°R
The plot looks like this:
Connecting node M2 to M1 we can calculate the speed and direction of relative motion...
We see above the speed of relative motion is 4.2kts over 45 mins.
The direction of relative motion is about 209°
If we add Ownships vector to the contact's vector we'll see they negate each other and the resulting true motion speed is zero. You'd follow the same exact procedure for a moving target, only the resulting vector would have a speed and direction of true motion that would not be zero. The "Add Unit" tool in MoBo was created so you could easily do the vector addition.