Do not book two cabins with a different high-level digit (like a 496 and a 502) because there is a fire wall and no adjoining balconies. Book like 502 and 506 - whatever is together on the deck plan with same high-order digit.
Also, if booking PLUS for the parents/adults, you can upgrade one of the 1-device Internet plans included in the package to a 4-device plan and then you have 5 total connections which is MUCH better IMO than expecting two youngers to be sharing a single connection.
Also, is the 20-year old going to be 21 by the time you sail (you said a cruise for next year). Anyway, there is no issue have those two on own booking.
There is even another workaround for this. Book everyone Standard. Swap cabins upon boarding. Upgrade adults to Plus on board Day 1. Same cost, but you have paid less travel insurance if you buy that and OBC can offset cost of Plus.