Well, it's hard to really make a concrete determination without a more precise mathematical description of how your rifts work and what you consider to be velocity, but the most probable answer is: there isn't really a consistent way to compare their velocities.
There are several reasons for this. The first one is very basic and comes out of special relativity, and it's simply the question "faster from whose point of view?" After all, if I'm sitting in one of the ships, the other ship is automatically going at least as fast as mine from my point of view, since I'll always see my own ship as sitting still. Now we can "fix" this problem by arbitrarily declaring some reference frame to be the one we measure velocity relative to (say, the Earth's frame), but we're still left with another problem from general relativity.
You see, in general relativity, velocity is an inherently local measurement; spacetime needs to be "flat" like that of special relativity in order to compare it. But the mechanics of general relativity dictate that space time need only look flat in small patches; overall it can have much more exotic composition-- much like how the Earth looks flat on a human level but if you walk in a straight line you eventually come back to the point you started at! This weird "bendy" global behavior makes it impossible to consistently compare velocities of separated objects.
As a more concrete example, imagine a pair of two dimensional beings that live in a world that's topologically a sphere. If it helps you visualize it, you can picture the sphere in three dimensions, but keep in mind access to a third dimension is not a luxury afforded to our flat friends-- the words "up" and "down" have no physical meaning to them. Now, say that they are at opposite poles of their sphere, moving at equal speeds along the prime meridian such that they will collide at the equator. What's their relative velocity? One approach is to say "why not look at their relative velocities in 3-D space?"
The answer is: many reasons. Now, there's actually a theorem called Whitney's Embedding theorem that guarantees we can always put this kind of locally flat, globally wonky type of object (called a manifold) into a higher dimensional flat space. But although the flat space allows us to consistently define velocities, it has drawbacks:
- Embeddings can get tricky to calculate for non-trivial space-times
- Most importantly, the velocities you obtain from this method have no real physical interpretation, because the 2-D creatures don't have access to a third spatial dimension. In fact, if you think about it, this method tells you that the relative velocities of the beings is zero, which seems like the least correct possible answer!
So, what if we give up on trying to use the global structure and instead concentrate on patching together local properties, much like how the manifold itself is intrinsically defined? Well, that's fine and dandy, but despite having a more physical interpretation, we run into the problem of multiple answers. For instance, imagine we set our reference frame to be that of the being on the north pole. If that creature is looking at the other along the prime meridian (which to him looks like a straight line), he'll see it moving towards him. But if he turns around and looks the other direction, he'll see it moving away from him! If you're into math, this is an example of the notion of parallel transport along a smooth manifold, which is a long winded way of saying that we're SOL if we want to compare velocities of distant objects in General Relativity. If you want to throw an even bigger wrench into the idea, just imagine what you would do if the sphere's radius was increasing with time.
The only reason we can talk about the speed of various space probes in our solar system is because on that scale, space is very nearly flat so acting like special relativity applies is a good approximation. But zoom out far enough and you can see weird things like galaxies receding faster than their light reaches us. This is often misinterpreted as them moving faster than light, but by now hopefully you've gleaned that such a phrase is utter nonsense since velocity and restrictions thereof are local notions.
Finally, to actually address your specific question:
Now, you may wonder why I spent so long talking about the problems of measuring relative velocity in general relativity. The answer is that it's even worse in the situation you describe! Like I said before, it kinda depends on the mathematical nuances of your rifts. But, as described, accessing additional spatial dimensions means that we don't even have the luxury of our spacetime being a manifold anymore, since its 3+1 dimensional spacetime abruptly glued onto a higher dimensional spacetime. So we no longer have even the notion of parallel transport to help us. Long story short, your question doesn't really have a well-defined answer.