It's not actually an I/O problem. An M1 macbook can drive a massive 6K monitor with no issues from either thunderbolt port. But try to drive two 1080p or 720p

It's not actually an I/O problem. An M1 macbook can drive a massive 6K monitor with no issues from either thunderbolt port. But try to drive two 1080p or 720p monitors. Impossible. The issue is the M1 only has two CRTCs to drive the video timings.