Back in the Core2Duo days I explored heat sinks that had a wide enough base to cover both CPUs and were short enough to fit in the space. I don't have the resulting spreadsheet on this computer, otherwise I'd just post it. I bought several of them to test, but never did. I don't think the data would be very useful now because all those heat sinks are off the market now, pretty much.
Things like the Thermalright XP-120. I was planning to build an adapter plate to use LGA-775 compatible heat sinks, but the XP-120 needs an adapter to be LGA-775 compatible.
https://silentpcreview.com/thermalright-xp-120-1st-120mm-fan-cpu-heatsink/
Interestingly, still available here:
http://www.xoxide.com/thermalright-xp-120-cpu-cooler.html
This just happens to be the only one I can remember without the spreadsheet. I think there was a flavor of Samurai (Scythe Ninja?) that was in the right size range, but I can't remember who makes the Samurai heat sinks and it was one specific model to get the large base.
I haven't checked the dimensions on this specific one, but sinks shaped like this one were interesting:
http://www.xoxide.com/scythe-bigshuriken2-cpucooler.html
I think that XP-120 came in a copper version. Something like an XP-120C or some such. But I had a bunch of notes on these heat sinks. IIRC, some of them needed teh fan moved from top to bottom. Otherwise they were too high.
Anyway, as with so many concepts in the legacy Mac space, I had an idea, did some research, bought some stuff, but never followed through...