With only one tanker, you can launch the tanker first and then launch the transporter, so it isn't any faster than launching a tanker and transporter simultaneously.

To go any faster you'd need more tankers as well. I don't forsee the tanker having commercial value short-term, so this would still increase the cost. With two tankers though, you can just have a full tanker waiting in orbit which is the fastest possibility, so there is no need of extra boosters in that case.

I agree with the premise of multiple boosters though. I wonder if a spacecraft could be made that could do both commercial earth orbit satellite missions, and serve as a tanker. This could finance a larger number of tankers. I think you'd need a 3rd stage on top of the tanker in this case to get the satellite in position. [EDIT: I mean either carry the extra fuel, or carry a 3rd stage + satellite, not both at the same time.]