thanks for all of the responses! this helps a lot.
Interest rate is so close, I'd suggest you take the "easy win" and pay off the $15k loan first and then start paying on your DH's loans once he's done with school and out of deferment. Unsubsidized loans while he's in school is a bummer as you're kind of treading water. If it were me, I might devise some kind of complicated system where I pay down both loans simultaneously, but that's not what you asked.
Out of curiosity, what kind of complicated system?
Since you asked, I would probably do what others here have suggested and figure out how much interest is accruing daily on that loan (yes, it accrues daily, and it sucks--when we finally paid ours off, there was a little calculator to figure out how much we'd owe on whatever date we were making the final payment) and make small payments every payday to pay down the interest plus a small amount of principal every two weeks (or however often you get paid), and then throw a bigger amount of money at the $15k private loan. Plus, it can be weirdly fun to watch your balance go down, even in small increments. :)
Something like this:
Paid every two weeks (assuming $500 each paycheck to make payments), so make two payments to each loan each month:
$75 to unsub loan
$425 to private loan
You'd still be able to watch the $15k loan go down dramatically each month, but you'd also be sticking it to the man by paying down the unsubsidized loan while in school.
I would also track how much in interest I would be saving by doing this, because I am weird and like knowing how much I saved. :)
But, I'd also add that federal student loans (assuming that's what the larger loan is), even unsubsidized ones, come with a variety of features that private loans don't--forbearance in hardship being one of them, so it's not a bad idea to just throw all your spare money at the private loan while you can, and then just start paying on the other one when it comes due. The forbearance thing could come in handy if you or your spouse isn't able to work for some reason and the income stops or slows down. I also got forbearance on a loan for a couple months while I was in the process of consolidating--they kept moving my loans from servicer to servicer after I graduated, and I had filed consolidation paperwork on one servicer while the loan was going elsewhere. It gave me a couple months breathing room while I figured out where my loans had gone and could re-file the paperwork. I could afford the payments (we did something very similar to you two, throwing an extra $1000 or so at our loans each month) but it was a good option.
I also want to say--this is absolutely awesome that you are paying these off aggressively. You get huge props for this! I graduated with my master's in 2009 and most of my grad school classmates are still paying on their student loans, while DH and I were able to pay off our total of nearly $54k in debt by 2013, and then we sent him back to grad school and were able to pay for it with our savings instead of taking on more loans. This definitely isn't the "easy" path, but it has made our lives a lot less stressful.