Well, I have just looked at the first roll of Neopan 400 with the two devs mixed. The negs are a touch undeveloped, but not by much. Comparing areas of equal visual density (I had some well matched skies), it is clear that the grain is less than FX-39 alone and less sharp, but a little more crisp than Xtol 1+1 and slightly grainier. It appears, based on very casual testing I admit, that the mixing of these two does indeed produce results about between the two. Dev times and film speed remain constant, although I will test again and check for density as it appears that this test roll was about 10 % under developed. That can occur with the temperature fluctuations in the rooms I use, which can result in convoluted measures to cool water down using the AC unit and then development taking place in a far warmer room (which I cannot get below 26 degs no matter what!). I was also rushing; however, the equally dense smooth skies tell no lies! They are about the same density so the 10X loupe test should me moderately insightful when comparing similar densities.
The mix produces good negs and I really cannot see any problems with mixing these two devs. I will reserve final judgement until I get to print identical test negs, however. I am hugely excited as this could be my perfect solution for full Xtol/FX-39 speed with TriX/Neopan 400 with more bite than Xtol and smoother grain than FX-39! I really expected this to fail miserably.
I will probably experiment further using a test clip which I will insert at the tail end of other rolls when I do Xtol and FX39 development. I will then do a third clip with the two devs mixed.