Odd distortion recording multiple tracks.

[Troy Winemiller]

Focusrite 18i20 3rd gen.

When I record a single track in stereo using a pair of interface inputs I get tracks as expected.

If I record the same sources but individual tracks simultaneously, they have a digital zippy distortion on both tracks. 

If I record ONE of the tracks there's no distortion.  If I record the same input to both tracks there's no distortion.

It's very evident using a bass as the source for the tracks.

What I've tried:

Multiple Focusrite drivers.

Multiple input pairs, and sources.

Internal and external clocking on the interface.

I'm not clipping anywhere.   I'm a Sonar user for decades, so I don't think it's a newb mistake.   I don't have another DAW to test.   I've recorded individual tracks in this manner a zillion times before and never had this issue.

Anyone else confirm?

Thanks!

Edited 33 minutes ago by Troy Winemiller