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In simple terms, if the film is under-exposed (e.g. rated higher), and equivalent amount of over-development is suggested. Or, if the film is over-exposed (e.g. rated lower), the corresponding under-development is suggested. Both color fidelity and contrast will be affected. Whether the result is still acceptable is a matter of personal taste. Note that the color temperature of the taking light can't really be compensated for in processing, but only via filtration.
Do both a and b apply to the same shot, or to two different shots?
If they are separate photos, then you would A: push A by 1 stop, and B: pull b by 1.5 stops.
Ralph is absolutely correct that the pushing/pulling will not help the color correction - in any case, your exposure will be off on both shots enough that they will be sub-optimal. You are going to lose some shadow details on A and highlight details on B, and the shadows on B will not be as strong. Can you live with the results you get is the open question only you can answer.
Guy,
In the first instance I would ask the lab to push the development 1 stop to compensate for being underexposed. The Velvia should be able to handle it. In the second case you have apparently overexposed the film by 1 1/2 stops. In order to compensate for overexposure you can request that the lab pull the development. However in this case the ability of Velvia to render properly when pulled has probably been exceeded. It has been my experience that Velvia 50 can only be pulled one-half stop.
Good luck