Yikes, just read the fine print on my reservation, and discovered that they charge $9.99 per day for an additional driver -- even spouses, presumably, since they specifically mention "life partners". Not such a great rate after all, so back to the drawing board.
(Maybe spouse is no longer the approved terminology? Man, it's hard to keep up.)

But persistence paid off: went through the Costco site (we are members) and found slightly better rate, still with Budget. And no extra charge for spouses.