that I tried to buy the carseat bag at Target and was going to try to put the two backless boosters in one bag, but it was an online only item and it was too late to order it. As a result, I bought jumbo ziploc bags (3 in a box for $5.24) and they worked great.
The online feedback on the carseat bags was mixed, and some said the material was flimsy enough that it ripped. I bought the largest bag size available and probably could have gone down a size. In hindsight I was glad I wasn't squeezing the seats in as I suspect that could contribute to the ripping some people experience with the carseat bags. The bags had tons of room, and I packed the extra bag in case one ripped but didn't need it. I marked them well with a sharpie (and included a thank you for good measure) but the gate checked items are given a specific colored tag, so it probably was overkill.
Because they were both backless boosters, they were very light to carry through the airports and I was glad to have the peace of mind that they were "superchecked" and not too far away from my control. On the way home we debated checking them with luggage and decided that it was easy enough to gate check so we did it again.
Next time, I will probably buy the smaller size, which will have four in a box, and mark two for the flight out with the flight number, and two for the flight back with the return flight number.
Thanks, again, for everyone's advice!