Swamp Trees Are Generated With Two Trunks Under The Ground

Absolutely All Swamp Logs Spawned With Two Logs Under The Ground

And Can Generate Under The Water Totally

When Spawned On A Ground Slope, It Spawns Only With One Log Under The Ground

It Can Also Happen With Some Birches (I Didn't Catch It, Because It's Rare For Them To Appear Like That, Unlike Swamp Trees)