If you use a phy that's mangetic-only you can do away with a single transformer per pair. I've designed boards for backplanes following standards that link the cards through ethernet over PCB and there were several appnotes for this as far as I remember. There are some PHY's out there that are ready for this, and some that can only do magnetic coupling. If you have to link the things through ethernet and want to do via capacitive coupling you can do so only if your PHY is ready for that. You'll need termination near each receiver side. At least in gigabit ethernet there might be some trace delays to take account for while doing this, depending on how flexible your MII's are.
If you can leave out the PHY's and just connect (R)GMII to (R)GMII then you can do it that way.
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