as far as i have seen and heard.... female turtles may or may not lay eggs... some do and some dont, i guess it all just depents on the turtles. but if your turtle does become full of eggs, she will possibly lay then in the water. but if she doesnt, she could become egg-bound and die. the better thing to do is to put her in some dirt for a few hours (this could take several days) and she will lay her eggs in the dirt. they obviously will not be fertile, but this will keep your females from becoming egg-bound. however, there is also a chance that neither of your turtles will lay eggs. i have also seen/heard people say that turtles will not start to lay eggs until they are like 5 or 6 years old. i hope this helps! 
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~ Evie 
Pets:
German Shephard {Dusty}
German Shephard/Collie mix {Shadoe}
2 RES turtles {Booger and Snot}
African Clawed Frog {Bingo}
Snail {Gary}