The real problem is that, in space travel, your speed determines your trajectory, and therefore two objects can't follow the same trajectory at different velocities. Precisely, a faster interplanetary trajectory will give you a more elliptic, elongated orbit around the sun, while a slower one will be more round (and I don't even consider the case of gravity assists, which makes the trajectory even more convoluted).
To summarize, if you send the fuel slow and the crew fast, their respective trajectories won't have the same shape. And if you send them on the same path, they must have the same speed, so the crew will never catch up with the fuel.
You could theoretically time the launches so that the trajectories intersect (a big challenge in interplanetary space already...), but at this time the crew and the fuel won't have the same speed nor move in the same direction. In order for them to rendez vous and dock, you need one of them to match the other's velocity and direction. Presumably, have the fuel match the crew's velocity.
But now, if they are in the same place at the same speed, that mean they are on the same trajectory. So you might as well have launched the fuel at the same speed than the crew in the first place, because all we did is only waste a lot of fuel launching in the slow trajectory and then correcting to end up on the fast one.
Consider also the risk that the crew and fuel miss their rendez vous because of some technical glitch. Having two vessels matching position at a few metres from another among billions of cubic kilometers of interplanetary void is no trivial task, and if anything goes wrong the crew is stranded with no fuel, and dead.
With this in mind it's much more sensible to have crew and fuel rendez vous in earth orbit, where the mission can always be cancelled in case of trouble, and send all at once on the fast trajectory, where your fuel will have to end up anyway.