First of all, UPN hasn't existed for nearly 7 years and KTXH has been My Network TV for almost as long.
Second, Fox for years has refrained from splitting their digital signal and programming subchannels, but that's about to change.
As I posted in another threat, 20.2 may soon be home to the new Fox/Wiegel movie channel and since they were splitting that signal once, they might as well do it twice and carry their in-house MundoFox network.
It could also be a way to get cable carriage of both the movie channel and MundoFox.
Third, since there is very little true Hi-Def programming on KTXH, any signal splitting will have negligible effect on the final product. Whereas, very little programming on KRIV is in standard-def so why mess with a crystal clear hi-def digital signal by splitting it to carry an unproven network(s).