what the set up was like and temps, and how they acted prior to dying. There are so many reasons.
Poor husbandry, parasites, its a shame. :0( But I would clean out the tank well, and start fresh with the remaining lizard.
Make sure he has, bugs, water in a shallow saucer, temps are right, and hope he does well. Were they ever checked for parasites?
Heres some brief info
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Jewelled Curly-tail Lizards thrive under dryer conditions, though medium humidity environments can also be appropriate. A 20-gallon-long terrarium with a deep substrate will work well for one or two lizards. The top layers of the substrate should be dry, and the bottom layer should be slightly damp. Full spectrum UV lighting is recommended. Ambient daytimeTemperatures should be about 85 degrees Fahrenheit with a basking spot about 95 degrees. At night the temperature should be in the low to mid 70s. It is best not to keep two males in the same enclosure. Jewelled Curly-tail Lizards should be fed a variety of insects. Some may accept fruit in small quantities.
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If hes thin or inactive you may want to have a fecal done at your vet to see if he has worms. Also males will fight, they are very territorial. Maybe you had all males????????
But in most of the things I have read they should be fairly easy to take care of and kind of hardy ! But I say that, not ever having them myself !
I sure hope you have better luck with your guy. Let us know how you make out! 
www.suncharmers.com
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Eve