For Orlando, specifically, you're not going to find a ticket that includes Disney and anything else. A first-time visitor will find the best overall package by choosing either to do an all-Disney vacation or a non-Disney vacation. (You could cobble together individual tickets to both, but there are better deals available if you commit to one or the other.)
You're staying at a Disney hotel, so I'd go with that. For people who choose the non-Disney option, there is an Orlando Flex ticket that you can buy through either SeaWorld or Universal that includes those parks (and, yes, Busch Gardens, too). I haven't bought that one in ages, so I'll defer to others on whether it's a good deal.
With two weeks, you could throw in a Universal three-day ticket or a SeaWorld ticket and go with a shorter Disney ticket (say, one week). But you'd have to buy those through the Uni or SW websites, you can't get them from Disney, obviously.
You could cut costs on that by staying at a universal partner hotel instead of their official ones but then you lose express pass access - I think you still get in an hour early though. You could also skip Universal and purchase Sea World Fun cards for the second week, which work like annual passes but are only like $80.
If you want to do a water park I suggest either a Disney one or Aquatica (seaworld's water park - separate admission required),because Wet and Wild (the Universal water park located off universal property) is pretty trashy.
I really agree with Robert's suggestion but if you're determined to fit it all in I hope the above helps.
Buy them online
Buy them at the Disney Store
Buy them at All Star when you check in
Buy them at the gate!
I would pick the first three!
I think the one week Disney hotel, one week Universal is a great option with kids. There is so much outside Disney to do.
I always stay near Seaworld and find that if I walk over at about 2 or 3pm I can buy my tickets with no wait or fuss. (and can take a sneaky peek in the souvenir shop just outside). If you only buy a one day ticket - head to the booth just inside the park entrance and claim your 2nd day free (or sometimes for $10)
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood