Thanks to everyone for the responses. Charlaquin and DM Dave1 affirmed the original problem. From everyone's posts, I see at least four solutions:
1) Tweaking the fiction/narrative to match the RAW. ("You had to hesitate a bit to get Ready to move together, so only Move, not Dash.) This is the most straightforward solution, yet probably least satisfactory given the karma of my group.
2) Resolving calls for simultaneous movement as a narrated non-combat ability check (outside of Initiative) or a chase scene. This sounds like it might be the most creative and acceptable.
3) As DM, keeping the PCs' "declared simultaneity" in mind throughout the round, even though it isn't apparent as the PC figures are sequentially moved on the grid. In other words, the PC who is "running behind the ox cart" does keep Cover throughout his turn, even though it appears the cart has moved way ahead of him. And the group of PCs madly Dashing "together" past the Goblin archer are randomly targeted by the DM, instead of the Goblin automatically targeting the PC who seems to be sequentially stick out first (or last).
4) Houseruling a campaign-specific
Dash Together special action (which can only be triggered by a slower ally's pre-signaled Dash, and which uses up the faster character's Action, Move, and Reaction), and possibly also a
Move Together special action (which can only be used when triggered by a slower ally's pre-signaled Move, and which, unlike a usual "Ready", allows the faster character to take their Action on their turn instead of blowing it on a "Ready." The faster character only loses their Reaction). This would only be about "readying" simultaneous allied movement, not about allowing Ready to be used for just any "Move + Action" (that would be opening a can of worms).
These guys just love running together - it's becoming a signature move of this party.
Thanks everybody! This a great and helpful community.