The times I have for HP5+ and FP4+ are 20 minutes at 20C, with 2 minutes initial agitation, and then 3 more 10 second agitations at evenly space intervals. I also use this same scheme with TMax 400. I'm a big fan of the acutance this produces.
For HP5+ this is somewhat similar to what I've been doing lately; I develop it for 18 minutes, 30 seconds initial agitation, then one agitation cycle every 3 minutes. With FP4+ this would result in excessive contrast in the negatives; I can't imagine that the same time would give optimal results for both films. 20 minutes is just way too long for FP4+; yes, it'll still scan OK, but there's no need to push the gamma that far at the expensive emphasized grain (which is further exacerbated if you scan through dense highlights).
Unless maybe you're exposing both the HP5+ and the FP4+ at E.I. 400, but I assume this is not the case.
Having said that, if you really find that the processing parameters should be the same for both films, then go ahead and bunch them together in the same tank. As said above you'll get quite wildly different kinds of negatives, but if that's Ok for you, then why not.