While the Pinnacle is approved for aircraft use in harness mode, I would encourage you to reconsider your plans. If I were you and I was traveling with a 6 and 7 year old, I would most certainly bring boosters instead of harnessed seats. Many people are under the impression that children are ALWAYS safer in a 5 point harness compared to a belt-positioning booster. However, we don't actually have any evidence telling us that. The truth is that, for children who are at least 4 years old (for skeletal maturity), weigh at least 40 pounds, and can use a booster seat properly (i.e. it positions the seatbelt well and the child is old/mature enough to not wiggle out of position or move the seatbelt out of place), as far as we know, a booster seat and harness are equally safe. Again, this is as far as we know. It is possible that there are some crashes/vehicles where a harness might have a safety advantage, and on the other hand there may be some crashes/vehicles where a booster might have a safety advantage. But IF the child is old enough, big enough, and can use a booster properly, it's just speculation on which is safer. In light of that, I personally would be perfectly comfortable having a 6- and 7-year old in boosters for the trip. You can't use them on the airplane, but you can pack backless boosters in carry-ons or just carry them on as-is.
Of course it is completely up to you what you do, but I just thought I would offer an alternative to hauling two Britax Pinnacles on a big trip that involves several flights and a lot of hassle.