@airsheep, you can't combine payments if some items are with Managed Payments, and some are not. Whether or not you're trying to use a coupon. I ran into the same problem, when I redeemed the $25 coupon.
The first item I wanted was not from an MP seller, and wasn't enough to meet the minimum spend requirement. I had trouble finding another item I wanted from a seller that wasn't in MP, but once I did, I was able to use the coupon to buy two items from different sellers, they just had to be sellers that still have their payments processed by PayPal.
I didn't receive an email or message about this coupon, so I couldn't check the terms and conditions (thank you @dana42scully for posting them). But I tried it in checkout and it worked (on one account anyway). I had to work from the error messages (not very specific) to figure out why it was insisting on splitting up the payment at first.
ETA: well, there's a message in my inbox about the coupon now, but it wasn't there yesterday when I heard about it, and made the purchase.
