possible solutions:
-use TS6 or higher and use the mirror tool
-find someone with TS6 to do this for you
-use whatever prog with mirroring capabilities and then step back to TS
-there are some plugins that let you join two objects connecting two faces with the same number of vertices (there are more than one of those, only one works with complex faces, and not always)
-move away on the side one of the halfs, extrude the internal face on each of the two halfs, so that the two geometries intersect, unify with booleans, connect manually the verts that weren't connected with booleans, weld the verts on the same lines between the two halfs, place all these verts exactly in the middle between the two halfs, move the two halfs so that they touch each other with the verts in the middle, weld the verts
-move away one the halfs, unify with booleans, join manually all the verts of the inner faces, move back in position the offset half, join all the verts
-don't join the two halfs, but leave them as separated objects, put them at the same hierachy level (grouped together and not with a light each) convert with pcs: if it doesn't crash, it will make a single object out of the two geometries, the two halfs will not be connected, but if it works, you will not notice that in game.
