YDD is currently a gymnast and ODD is a former gymnast, with her own new sporting and non-sporting events now, but certainly not requiring the level of travel and time that gym does. We have done a few things to make this feel balanced.
We don't make ODD come to meets if she will have to miss school or her own practices/activities. We give her the option to come but do not require it. If she can come, we all go together (mostly if it is drivable or if we have family to visit near the meet location or if it is a fun place to visit and we can turn it into a mini-vacation) and if she cannot, one parent goes and one stays home.
We do our best to switch the parent who goes so that our ODD never feels like "well, mom always goes with YDD and never hangs with me". This is somewhat tough to navigate since dad can't do meet hair and since YDD prefers me to go (I'm calmer!). But, for the sake of our ODD truly knowing that we place equal value in everything she does and for the sake of our YDD truly knowing that the world does not revolve around gymnastics, we switch off.
We have friends where both parents go with the gymnast and then another family member takes the siblings to soccer games, swim meets, etc and I always feel horrible for those kids and worry that it will eventually result in some sort of complex for them. Because really, to them, their local soccer game is as important as the gym meet, as it should be.
I strongly agree with many posters above. We don't really celebrate each meet per se. Awards, eat, usually the "celebration" is a DQ Blizzard that both girls get, and then home. And we do the same for our ODD at her events.