I'm trying to calculate the Tsiolkovsky delta-V for a parallel stage like the Space Shuttle with greatly different ISP. How would you calculate the "effective" ISP for that kind of arrangement?

Isp is a ratio of prop consumed vs thrust produced.

You can add the solid + liquid prop consumed, and add the solid + liquid thrust produced. The ratio of these two sums gives you a blended Isp.

Unfortunately, both the solids and SSME were throttled, which makes this more complicated for Shuttle. Any change in the proportion of thrust from the two systems needs a separate calculation of the blended Isp for that portion of the flight.

But, you may be able to get away with just averaging prop use and thrust over duration of solid operation.

Cheers, Martin