I've traveled with DS to Australia 3 times since he was born, so 14+ hour flights, and at least 3 domestic flights a year. He's now 5yo. From my experience, I would take
snugride with snap n go for the baby. You can install the snugride in the car and the plane without the base, so leave the base at home. As a pp said, practice how to use the locking clip.
For the 2 yo, bring a scenera or an Uptown. Much much much lighter to carry. I've strapped it to a McClaren stroller - bungee cord through the forward facing belt path and strapped DS into the car seat. Perfectly stable and easiest way to get around the airport. You'll need to know how to use a locking clip with this seat as well.
Do rethink checking some luggage. Trying to get on the plane is the worst part of the trip. You have to stop at the door to the plane to collapse the strollers as they will be gatechecked. Then, you and your wife have to carry 2 carseats, pull/carry carry-on bags, plus carry the baby and either carry or herd the 2 yo down the aisle to your seat. And you seat might be a LONG way down the plane depending on what door they use for boarding. Plus, the aisle on plans is very narrow. I've walked the length of the plane and had to carry the carseat over the top of all the seats, as if I carried it in front of me, I hit every single seat as I went past. Some carry on bags can't be pulled down aisles. I know this is an international flight and should have more room, BUT in the last 10 years I've noticed the aisle on international flights get narrower and narrower.
We limit our carry on to a back pack each, and DS's car seat and folding luggage cart as he doesn't need a stroller anymore. That's it. And it's still a pain to get it all on the plane. My DS is nearly 5 yo, so for the last 2 years he's been carrying his own bag and can walk on his own, so that has helped.
Also pack LOTS of extra diapers. We've been stuck on the tarmac for 3 hours before take off, and then DS got diarrhea half way into the trip. You can't use the DVD player for take off/landing - so you need other toys etc to distract your 2 yo. And then you'll be spending a lot of the flight doubled over trying to pick things up from the floor.
Check on when they serve meals on the plane. Our flights to Australia leave at 11pm or so, and they serve dinner at midnight and then nothing for another 10 hours or so, as they serve breakfast on Australian time. We make sure we eat a meal at the airport, DS then falls asleep as soon as we take off as he's exhausted from staying up late. He wakes before they give out breakfast, so I always have lots of snacks to tide him over. Request the kids meal.
It's exhausting to travel so far with little ones, but it is worth it. And the more we do it, the better we get at it, and thankfully DS is older and can cope with more.
Have fun.