I dont know if you mean they are socionics dual or enneagram dual, but they are definitely enneagram dual, cover each other's weak aspect in every sense. But if we cherry pick for them, their biggest downside would be that they are too positive as couple, would over look some problems and sweep it under the rug. Again other problematic areas in that duo, that there is too little competency, again problems and conflicts could be remain unresolved sometimes due that. On Hornevian aspect, none of them are really efficient at knowing when to back away.
268 Tritype:
Harmonic - 2X Reactive 1X Positive Outlook
Hornevian - 2X Compliant 1X Assertive
Object relations - 2X Rejection 1X Attachment
739 Tritype:
Harmonic - 2X Positive Outlook 1X Competency
Hornevian - 2X Assertive 1X Withdrawn
Object relations - 2X Attachment 1X Frustration