Personally, I don't see any problem with doing that, but I would keep the geraniums a few inches from the peonies so there will be better air flow to prevent any diseases or other problems caused by moist, stagnant conditions. And it would be a good idea to cut back the geraniums adjacent to the peonies in late autumn, so the emerging new peony growth in spring will get sufficient sunlight.

Since peonies like full sun and G. macrorrhizum doesn't, I'd use G. 'Biokovo' or 'Karmina', much like G. macrorrhizum, but smaller and good for sun. I grow all three and am very familiar with them.

The geraniums (for me) are of lesser stature than the peonies...thus no issue.

Ok sounds like the consensus is that this should work. I'll have to look into the full sun/shade thing. I was thinking G. macrorrhizum worked in full sun too. I'll have to check on the G. Biokovo, smaller is probably better with the peonies though to allow them more air flow. Thanks for the feedback everyone.

Yes, this is a good combination. "Biokovo" is a very tough, extremely useful groundcover, I can't recommend this one enough.