So you have 2 absolute positions p1 and p2. Assuming they have same base and tool then inv_pos(p1):p2 is the shift/distance in the tool system. If you want to it with repect to base then p1:inv_pos(p2) is the shift in the base system.
This can also be applied if base and tool are different. If you use forum search you can also find how to use the geometric operator : in combination with inv_pos to change positions to different bases or tools.
If you have already absolute positions I do not understand what does not allow you to use them directly.
Fubini