I have 2 sliders... and the lady i bought them off of said that they are much happier when they are with others. but she also sold me a 'death bowl' and told me that they could live in that with plenty of room for at least a year! needless to say she was WRONG! now my turtles seem to get along 'fine' so far (knock on wood)... but i have heard MANY stories of other people who have had turtles that just get sooo territorial and literally bite each other and leave wounds! deep wounds that need to be taken care of by vetinary assistance! so honestly, the choice is up to you... but just remember that if you do decide to raise two RES together, they 'may' not get along, and you would need 2 different tanks/containers for them! also, if you get two RES they will OBVIOUSLY require a much larger tank than if you would just have one! hope this helps...
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~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}