I have had the pleasure to meet and talk with both, together (inside tips: the healing surge mechanic is mearls favorite
4E inovation, and Jonathan Tweet is not an eazy DM).
I wonder if the key word here is "mechanical". Mearls will be a designer now, originating ideas, but the focus is still on mechanics. Heinsoo will be out of the mechanics box, and able to design a wider range of things (or maybe not worry so much about how everything works together?). This would fit, based on my own first impressions.
EDIT: though Heinsoo also mentions "team leader"...on this point, I couldn't really say anything.
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