Both my wife's phone and mine are on my TMobile account. I assume if I go to 'Manage data and add-ons’ at the plan-level (we have Magenta), buying an International Data Pass will make high-speed internet available to both our lines, w/ the 15GB limit being the sum of our downloads.
This isn't explicitly stated in the international topic pages, but I hope it's true, as the data pass seems much less than buying individual European eSIMs would total.
Thanks.