While these two sensors are related to how the engine runs, their function and what they provide for engine management are completely different.
MAP (or Manifold Absolute Pressure sensor)
The MAP provides the computer with information as far as the density of the air. This tells the engine how much air is actually getting into the engine. This, along with the Mass Airflow (MAF) sensor (if so equipped) and O2 sensors, tells engine management how much fuel to disperse into each cylinder to keep the air/fuel ratio somewhere near stoich so the engine will run at its best with fewer emissions.
TPS (or Throttle Position sensor)
The TPS is basically there to provide the computer with the driver's input. What does the driver want to do? Larger TPS reading provides for more load to be put upon the engine and for the vehicle to go faster. It can also indicate to the management system if the need to downshift the transmission is needed to provide the vehicle response the driver is wanting. Of note, with most vehicle manufacturers moving towards "drive by wire" (no direct connection between the throttle pedal and the throttle body), there is no need for a TPS. Since the computer controls the throttle, it knows already where the throttle position is at, because it is directing the show.
Mind you, these are the general reasons for having each of these sensors. While you could possibly infer throttle position due to manifold pressure, having both sensors allows the engine to be more responsive. If running just off of the MAP sensor to determine these things, the computer would always be reactive and trying to keep up with the demand. There would have to be large assumptions built into the programming and I'd assume a larger more powerful engine management system would have to be applied to compensate. Having both sensors there gives the computer the exact want of the driver as well as the amount of air flow going into the engine to provide the driver with a much better driving experience.
It should be of note, there is also the same argument made for having both a MAF and a MAP sensor installed on the vehicle. These two share a lot of the duties to help with engine management. Many GM vehicles were produced with both sensors (and still are). Without a MAF, the engine management can run in what is known as speed density mode. While this mode works, having the MAF in place provides a more precise measurement of incoming air for the computer, and thus better fuel management, economy, and lower emissions. It does provide a restriction in the intake tract, though, which is the tradeoff.