This is their mating dance. Likely you have two males displaying for eachother here. Do they both have fairly long front claws?
If you have two boys and are introducing a 10-11 inch turtle to them it IS a female (males don't usually get that large). You could really upset the apple cart by doing this. Both males will try and mate with the female and could very likely stress her by constantly being on her. Also they could start fighting each other to mate with her. You would need more than one girl and then there is the issue of space when housing that many fairly large turtles together. The boys are getting on just fine with eachother now so I would leave things alone unless you are prepared to invest the $$ time and space to create a larger habitat to house several turtles.
If you are willing to do this we have plenty of Sliders at Turtlehomes available FREE of charge. All you need to do is to get your proper environment set up, take a photo of it, go to turtlehomes.org and fill out an adoption application. Then mail it along with the enclosure photo to the mailing address listed on site and you will be notified if you are an accepted adopter. You will not even have to pay shipping charges on the animal.
Steve