I think having 2 in the SAME sport would be slightly easier. At least at our gym, most of the competitions are attended by as many levels as we choose for that particular meet, and includes MAG if possible. The lower the level, the fewer meets you attend. The boys DO usually go to a few different ones than the girls, so that would be slightly harder (having 1 boy/1 girl) but the coaches try to take that into consideration if we have sibling pairs on team.
In the lower compulsory levels, the cost would be ok and possibly you'd get a tuition break. Moving into Optionals the cost would skyrocket though!
I am lucky enough to have one fresh Optional gymnast and one serious competitive dancer.
. Both sports are extremely expensive (dance is even costlier than optional gymnastics!!) and, lucky me, the seasons run at the same time. I only have a couple of serious conflicts this year, I have solved one of them by sending the older one with her friend for the weekend which stinks because truly I LOVE being there and watching her dance (and it's a big competition) but there's no way I can be in 2 spots several hours apart at the same time. All our competitions/meets are minimum 2hr drive, most 4hr and (again, so lucky, lol!) this year we are supposed to do 2 flyaway meets (which of course coincide with either competition or rehearsal for my older one, so she wouldn't be able to go) . Going from the first weekend in January to the second week in May I have exactly TWO weekends I will not have to travel/stay in a hotel, and the first "free" weekend isn't until late March.... I do it mostly myself, DH is supportive but has to work. He's willing to go to meets, but not dance competitions. We also have pets that would need to be kenneled for overnight trips, so most of the time he simply stays home and I make it work.
We couldn't say no when YDD was offered (and wanted) a spot on team. Not fair to allow one kid to pursue her dream and not the other.....
ETA: for every day, they practice at similar times and both are old enough that I can simply go back and forth dropping off and picking up, and on the nights I work late at the gym and my dancer is done before me, luckily we have a family at dance who live close enough to carpool and they take her home. They both have practice every day/week except Sunday, well I take that back, sometimes dancer has rehearsal on Sunday's too....