Hello and welcome :
He /she you can hope for but still to small to tell sex. Until they are sexually mature it's a guess. But they know what they are and chances are you could have two males with the aggression. You need for every 1" of shell 10 gallons of water area. So just say both 3" that's 6" of turtle x 10 gal=60 gallons water alone. You may have to separate permanently ! 10 gal way to small even full with an ATBA ! If you get the 90 gal how much water going into it ? Half full so they don't escape , that 45 gallons better than the ten but for two turtles ??? 90 gallon for "one" would be a nice home if a male. Could work if you offer a lot of hiding spaces "A LOT" so they do not always see each other. Tank divider but then you need two basking areas. Make sure you feed them separate so no fighting over food. Turtles grow fast adult male RES's get 9" that's 90 gallons water area alone and female's get 12" that's 120 gallons water area alone. They also live a long time around 40 years or more.
Please do not sleep with your turtle you could suffocate it !
Not trying to discourage you , then you need a proper filter for that larger tank. Most filters are rated for fish gallons ! Turtles require 2-3x's more flow than fish being so messy ! A 90 gal filter will not work , would need between 180-270 flow for one turtle and if two in same tank even more . Some say 3-5 x's flow rate. Not cheap start saving your allowance and ask for turtle things at Christmas. Turtles are great pets and what a personality each have.
I'm retired and been doing this over 50 years , to save me money I use Rubbermaid stock tubs. Just an idea of what can be done with a stock tub.
https://youtu.be/VvD7-StKjCQ