You have a few options. Usually I start a VFX shot by thinking of the camera movement. If I move the camera what problems will that cause?
From your description you have a tracking shot with two actors. The camera tracks the first actor as the second actor enters the scene, then camera comes to rest and the second actor continues to his marks at normal speed while the first actor speeds up. Shooting two plates will give you control of the timing of the first actor but, considering the camera move, will require you to precisely match the timing of the two tracking shots so the perspective between the actors is consistent. It doesn't matter whether or not the shots are on a green screen. For the perspective between the two actors to match the camera moves must be identical in camera angle and tracking speed.
If, on the other hand, you shoot the entire action in a single pass then you could use rotoscope to split the shot into two pieces where the camera stops moving and then adjust the timing of the first actor. This will work easily only if you make sure that you do not move the camera at all once the tracking ends.
If the actors do not overlap then the second option is a piece of cake because your roto can be a very simple rough mask. If the actors do overlap in the frame then your roto work will have to be very precise where the actors overlap and you may need to do some background replacement to fill in holes.
If you decide to do it with two plates then you will most likely need to do motion tracking. stabilizing, and re-timing of both shots to make them match up. If the camera is moving when you need to speed up the first actor you will have to do some motion tracking.
A last option is to shoot the first actor on a green screen and the second actor as a background plate and then track and stabilize both shots to try and combine them. Here again, if the speed and angle of the two shots you are combining are not identical or at least close enough that you can easily fix them you are going to be in for a lot of fiddling around to make the shots work.
If it were my project I would shoot both actors in one take and then split the take where the first actor needs to be have the speed change and do some roto work. That seems like the simplest approach.
That completely makes sense, thank you for the response!