Late to reply to this, but this is two pronged. Long story short, you arent being screwed out of an early booking opportunity, as long as both cabins have valid castaway status in them, but it will need to be done via phone call.
No one, regardless of using placeholder or not, can book 2 cabins from 1 castaway account log in during the early booking oppertunities. You would have to log in to your account, book, then log into your husbands and book (Or have it logged in on a 2nd device and book at the same time).
Then the second complication is that no one with 2 cabin placeholders can book or modify the sailing date of these online. If you had only 1, you'd be able to use it to book online.
If you (or TA, in your case) call during your castaway early booking opportunity, as long as someone with the valid castaway level is being assigned to each cabin, you can use the placeholders for the discount and get the early booking ability.