Indeed. The only possible way transits could take place in the Uniplanar model is if the planets were at the same altitude... But if that was the case they would have to be tiny! When either planet transits it's seen as a small black silhouette, minuscule in comparison to the sun, and so the planets would have to be less than a mile in diameter. This is plainly not the case!
I'd say it completely debunks the Spotlight Sun model which is heavily valued by FEers.
Challenge?
No "challenge", the maximum angular size of Venus is about 60" of arc, or about 0.032 the angular size of the sun. So yes the size of Venus would have to a bit under a mile.
Another interesting question is why there are:
- regular transits of Venus and Mercury,
- many more transits of Mercury than Venus,
- NEVER any transits of the other planets.
Also it is interesting that both Mercury and Venus show phases, somewhat like the moon, and
Venus varies dramatically in apparent size, Mercury less so. etc, etc.
So much we can learn from these "little lights in the sky"!
On the FE it just happens! On the heliocentric globe these are very simply explained.