I've been trying to come up with something sort of in the middle, but while the systems are almost perfectly compatible, at this point they are
not quite so.
The biggest problem mechanically is that even if "civilization has fallen" such that there is no purpose to Vehicular and Technical skills, there is still the Scientific skill category, which isn't in HARP Fantasy at all. Moreover, characters developed in HARP SF had 3 more categories they had to spread their DPs out among, which affects how much power/skill/etc. is "enough" for a beginning character. Thus either HARP SF characters start slightly more powerful than HARP Fantasy characters, or they start feeling slightly "underpowered" for their own "native" setting/mechanics assumptions.
The other major issue is as Marc R noted, trying to get a balance between technology and magic. It gets worse still if you include psionics
and magic both. The way around that as a GM is to tweak the setting to where technology is difficult and dangerous, tweak the magic/psi mechanics so that magic/psi/both are difficult and dangerous, or both. This way you can get powerful magic/technology in the game, but you
won't get it unless people are really desperate.