The drug combination index is commonly applied on inhibitory effects. Inhibition cannot exceed 100% and is minimally 0%. 

Stimulatory effects have to be assumed to be indefinite in absolute figures, but have an agent and model related maximum, and you observe a ceiling effect.

The easiest way to look upon combination of stimulatory effects is to determine the dose-response relation for drug(a) and drug(b) alone. If you assume simple linear additivity, you can calculate a hypothetical dose response curve for additivity. You can determine the de facto dose response curve for the combination and compare it with the hypothetical addition effect.

However, this comparison might not reflect the development target. In certain systems it might be a target to reach stimulation by a combination just beyond the max stimulation of either drug(a) or drug(b).

If you want to use the tool of combination index even for additive stimulatory effects, one could consider to determine the individual drug dose response curve, estimate the maximum stimulatory effect (e.g. by plotting vs 1/Drug, conc (or dose) as the intercept ) and calculate the real stimulatory effect as % of maximum stimulation for each drug. This is a parameter which is normalized to the individual max stimulation.

This dataset would allow to be used by the algorithms for calculation of drug combination index. 

However, in contrast to the understanding of combination of inhibitory effect which shares a common maximum endpoint (absolute and relative 100% inhibition, single drugs and combination), the relative and absolute max effects of stimulation are not absolutely identical between the single drugs, and the relevance of the absolute stimulatory endpoint of the combination in case it is larger than any individual drug, or like the linear additive effect or even supra-additive has to be discussed very carefully.

However this approach might be a tool to compare different combinations by a numeric parameter like Fa. However, this approach would need to be combined with a thorough look upon the absolute stimulation (single drugs and combination) for an  interpretation.