Follow-up to 12/18/20 Question about Feeding 2-Gang Receptacle Boxes with MC 12/4
Since each circuit has its own yoke of receptacles, you're fine
The rule that Harper's referring to is in NEC 210.7:

210.7 Multiple Branch Circuits. Where two or more branch
circuits supply devices or equipment on the same yoke or
mounting strap, a means to simultaneously disconnect the
ungrounded supply conductors shall be provided at the point
at which the branch circuits originate.

Since you are giving each circuit its own duplex receptacle, and thus its own yoke, though, this rule does not apply to your situation, so you can leave the two breakers un-handle-tied.  (Note also that since you're dealing with 120V AC, 406.5(J) does not apply to you, either -- that rule's generally only a concern when dealing with 277/480V systems found in industrial work)