You can use a single account. Make a joint submission when travelling together. When travelling solo just select the appropriate person when making the submission, (just check the box beside the appropriate person).
Each submission has to be done exclusively on either the web page or app. I haven't tried with two separate phones, but I suspect the app can be on separate phones and use the same account. But a submission started on one phone must be completed on that phone.
If I understand the plans:
Joint submission within 72 hours before crossing for flight to Europe.
Presuming separate return flights: Husband only submission within 72 hours of returning flight. Wife only submission within 72 hours or returning flight.
If both returning on same flight, joint submission within 72 hours of scheduled departure time.
Husband submission within 72 hours before crossing to pickup wife.