Yes, I have done this - stacked LIT UP with both RPM and BLACK CATS. How one would dose depends on that individual - everyone has their own "sweet spot" and you need to experiment a little to find it. Try starting out at a scoop of LIT UP with half your usual RPM dosage, and then, if that's not giving you what you want, use a full scoop of LIT UP and your full dosage of RPM.
~Rosie~