You "apply" for a rating or certificate by showing up at the examiner with proof of your written test and taking a flight test -- so of course you can only do one at a time! But I think you want to know if you can train for both airplanes and gliders at the same time, under the authority of the same student pilot certificate. Yes you can do that.
However I do not recommend it. They are very similar of course, but there are enough differences that while you're learning to fly, you should just pick one and get your certificate. Then learn the other.
If you want to fly both airplanes and gliders, there is no particular order that is better than the other. I fly both airplanes and gliders; I happened to do airplanes first because there was no convenient glider field close to where I lived at the time. Now I fly both and I feel they complement each other and it's great!
But while you're learning, pick one and get it done; then do the other one.
(This may only be correct with the FAA in the US. And to be fully FAA-pedantic, Airplane and Glider are "categories"; "single-engine land" is a "class" that is available in the Airplane category. Thus ASEL is category-and-class, but Glider is just category.)