If the A and B isomers have sufficiently different spectra so that the absorption of B is independent of the concentration of A, then the total of both isomers would be measured at the isosbestic point, where both species have the same extinction coefficient and the concentration of B would be measured at the B specific absorption wavelength based upon a calibration with only B being measured. If there are not any wavelengths that A and B have negligible contributions to the other then is will be necessary to use at least two wavelengths where each isomer has a different extinction coefficient and the solution to the two equations and two unknowns would need to be measured and solved. This assumes that there are not interfering absorption in the isomerization reaction solution.

I think you can do this by applying the developed method HPSAM (H-point standard addition method) for spectrophotometric determination of the two components if there is a spectral overlap between them based on the principle of choosing a pair of wavelengths