A 1-mm gap in a ferrite core is huge. It is like having separate cores. You might ask, why hold them together at all? (That is a small exaggeration.) If you just want to physically hold them together in some loose approximation to each other, hot melt or an epoxy will work.
If you want to couple them closely, consider that ferrite is like a glass, so I would treat it like glass. Polish the parts that need to fit tightly together. I can't suggest anything for high volume production, but for my low volume experiments, I use 3M abrasive papers (actually, they are not paper; they are on a polyester film). They come in grits <6000 to fractional microns. I lap the parts with water (a drop of soap in 100 mL helps) on a flat glass plate. Then the parts almost stick together like lapped measuring blocks. A very tiny drop of the most liquid CA adhesive you can find will bond them forever.
John