Hands down these two OSR books work like retroclone peanut butter & jelly. While Cha'alt has Crimson Dragon's Slayer D20 built into it. AS&SH has this plug & play ability with the setting book that makes it just that wee bit more Sword & Sorcery purple weirdness. In fact drop in Tom Moldvay's classic B4 The Lost City & you could have a full on mini campaign of solid adventure for months or years!
The factions in B4 The Lost City line perfectly up with all of the material in Cha'alt & the dangers of the cults of Zar'gon or its children are right in line with the horrors presented by Venger's world. In fact the setting of the lost city could be found within the wastelands of Cha'alt. The small cities of Cha'alt might do a fair trade in some of the artifacts recovered from some of the ruins scattered across the Lovecraftian planet.
Hyperborea might have Cha'alt as a satellite in orbit around one of its lesser moons close enough to be legend but far enough away to only have Cha'alt residents make a forced landing or two.
Zar'gon & his ilk might have some claim on Hyperborea but B4 The Lost City might have a gateway or two onto the demon worm planet of Cha'alt. The possibilities here are dangerous & these cults could become a real world problem for the residents of both Hyperborea & Cha'alt itself. Adventurers from Hyperborea stuck on Cha'alt are going to have to make their way in an alien world that they hardly understand. I would use the starting equipment from AS&SH for PC's trapped on Cha'alt. The learning curve is going to be huge for those stuck on the world of the black pyramid.
The magicks of Cha'alt itself are intense dangerous & very violent! Travel between the two worlds would depend upon the whims of the Boreas winds. And the attitudes of the gods. In the end there's some real fusion possible with the right group of players.