Question: Behavior of "Tolerances" package

Hello
Yesterday I was playing around with the Tolerances package in Maple 12. To familiarize myself I with it I tried to work out the equivalent resistance of two parallel resistors with tolerances.
R1:=80.0 +/- 4.0 [Ohm]
R2:=120.0 +/- 6.0 [Ohm]
Maple returned: Req:=R1*R2/(R1+R2) = 48.5 +/- 7.22 [Ohm]   (rounded values, there is some weirdness going on with this also.)
The right answer should be 48 +/- 2.4 [Ohm].
After some investigation I figured out that the "Tolerances" package apparently allows the same variable used more than once in a formula to be simultaneously at different values within its tolerance limits.  Physically, this is not correct, a resistor can't be at its minimum and maximum value at the same time, for instance. This greatly limits the usefulness of this package for tolerance analysis. Is there a way to force several instances of the same variable in a formula to change in "lockstep"?

Thanks for any help!
Andreas

P.S.: It would be great to be able to paste Maple input directly into these posts.