Laid in pairs?

So I've found an egg in my leopard geckos' cage. Unfortunately, the female did not lay it in the laybox, and I just now found it and it's dried out.

I tried searching everywhere for another egg, but did not find one. Do they usually lay them in pairs like crested geckos? Is it possible for just one egg to be laid?

How can I get her to be more apt to lay it in the laybox? I have moist...but not wet...EcoEarth in there. I did have it uncovered, but I just now covered it with a hole in the lid, thinking she might feel more comfortable going in there. Should I put it over the heat mat or at least closer to it? It's currently about 8-9" away from the heat mat and doesn't seem to get any heat. Should I put another heat mat underneath it? It's been rather chilly at night lately, so the four geckos seem to stay near the heat mat. Temperature on the heat mat area is currently 89*F.

Leopard geckos can lay 1 egg instead of a pair, it's normal for them. They don't always carry 2 eggs at a time, nor are 2 eggs always ready to be laid at the same time.

Sometimes geckos will lay their eggs outside the laybox, there's not too much you can do about it. A lot of the time this is due to the eggs being infertile, and they somehow know it so they lay it outside. Other times they are good eggs and for whatever reasons they just choose to lay them where they are.

If you have a laybox with cocofiber, that's an ideal substrate for layboxes. I do find it better to put them over warm spot, doesn't have to be directly over the UTH, but it should be over something warm. I find that my geckos use it more when it's warm rather than cold & damp.

Sorry to hear the one egg was all dried up!
Even though it was most likely infertile.