Since Pogo leaves 4 prints per meter, then over 6000 meters it would leave 4*6000=<<4*6000=24000>>24,000 footprints on the soils of Mars.
Grimzi leaves 3 prints per 6 feet, or 3/6=0.5 footprints per meter.
Then over 6000 meters, Grimzi leaves 0.5*6000=<<0.5*6000=3000>>3000 footprints on the sands of Pluto.
In total, the two creatures will leave 24,000+3000=<<24000+3000=27000>>27,000 footprints.
#### 27,000