I've done this, but not in Glorantha. I got around the timing problem by running both groups as "episodic": "three seasons pass, and then one day...."
And in terms of interaction between them, they usually stayed some distance apart, and heard about each other by Rumour. Which was particularly amusing for the guy who played in both groups for a bit, because the rumours weren't very accurate :) We did once have one big session with everyone from both groups attending, and the only problem there was me trying to GM a dozen or so players at once.
Reusing scenarios: well, I did. A lot. Sometimes I renamed and re-located the village where they happened, but that last big session did feature three PCs who all had identical magical swords :)
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