STL files are 3 dimensional, of course every 3D object has two sides, lots of them depending on where you define the plane that separates the object into "sides".
If you look for artwork to cut you will usually have difficulty. Take a statue for example. You split it front and back, so you have a front "side" and a back "side". Since most stl files are designed as true 3D objects, the thickness needs to be altered as a rule. Further, most 3D objects have lots of undercuts.
A real problem is cutting something like a vase of flowers. Attempting to adjust the heights with something like that can cause brain damage.
The falcon is a good example of why a lot of STL files do not work very well:
Notice that radar antenna. Undercut time. Then, if you tilt the model to avoid it being real obvious, things go really bonkers.
A bit of elbow grease will be needed, but there is plenty available online.
Different people will want different things.
I have a nice 3D of the Nautilus, I have a really nice Whale, and a really nice Osprey with wings spread out. The osprey will not work in Aspire, but I have other options for that. I also have a 1965 ford mustang.
I had to spend time and effort and search for those things.
I think as time moves on there will be more are more 3D models becoming available, because there are more people getting more sophisticated and more machines with capability.
I cannot think of all the places to look, but GRABCAD is one place I have a couple of models from.
You can bring it into Aspire and/or other 3-D software and slice the Antenna off and make two stl files. However the antenna would be easy to replicate by itself and then glue it on afterwards. Yesterday I finished an AT AT Walker that I got the idea from Instructable.com. I think I will try the Millennium Falcon using the double sided feature in Aspire V9, and see how it goes.
2 sided stls are abundant on the net. Thingiverse, My Mini Factory, Grabcad, turbosquid,,,,,,,,,,just to name a few. You can machine them how you like just takes a bit of work to ready them for cutting in Aspire. The attached was a full blown 3d model stl I just chose to base relief it. Google is your friend when it comes to locating models of all types.