There are a few things that are not possible in relativity -
1) Any massive body can not move at speed of light.
2) Even if it did, it could not slow down slowly.
These are probably limiting cases in relativity and I would not mind also considering them safety net for relativity.
3) Even ignoring above, any two observers, both moving at speed of light in same direction wrt a third observer, are not moving at all wrt one another. So if one of them slow down a bit, will see the other left behind (or pass ahead) with that bit of speed.
Only thing that we understand, and have formulated till this time, is relativity. We do not seem to have means to understand (let alone formulate) absoluteness, even if there was such a thing. But various paradoxes indicate, there could be. So, keep thinking.