"So is it saying if either spouse has a 401k but the other doesn't, the combined MAGI limit for deducting at least some of a contribution is $214k, and if both spouses have a 401k, the combined MAGI limit is $129k?"
No, that's not quite what it says. When only one spouse is an active participant in a 401(k), the $129k MAGI limit applies to the spouse who is an active participant and the $214k MAGI limit applies to the spouse who is not an active participant. As you said, though, if both spouses participate in 401(k)s, both spouses are subject to the $129k MAGI limit.
IRAs are individual accounts, so with respect to IRA contributions, so "You" in the IRS references refers to the individual making the IRA contribution.
