If you want to precalculate the value, I imagine it's possible, but I don't know how.

If you have the two lenses and want to determine through experimentation what the effective f/stop is, that's doable.

First mount the one of the lenses. Put camera in manual mode, and take a shot of something, preferably a neutral gray card. Something that is a solid color would be best. Set the shutter speed so that you get a good exposure.

Now add the reversed lens. With the same fixed ISO, take shots of the same subject (card) and with a range of shutter speeds. Because this will now be a macro shot of a small area of your original subject, that's why I suggested a solid color. Be careful that you are not blocking your light source with the lens.

Then compare those shots and find one that matches the tones of that first shot.

The difference in shutter speeds will tell you the additional stops of light beyond f/2.8 that you've lost by adding the second lens.

So if the original shot was at 1/60, and you have a shot with the two lenses at 1/15 that roughly matches the brightness of the original, then that's two full stops (1/60 --> 1/30 --> 1/15), so your effective aperture is f/5.6 ( f/2.8 --> f/4 --> f/5.6 )