Depending on which slicer you use, you can probably merge them as an assembly. I use Orca Slicer and after loading a model I can then right click on it and add an STL to it. Then it’s a matter of positioning and lining them up. Once done you can slice it as one model.
I believe you can export as an STL from Orca Slicer, though I haven’t done it myself. You’d basically merge the STLs in Orca, export, and then open the exported STL in the slicer of your choice.
Depending on which slicer you use, you can probably merge them as an assembly. I use Orca Slicer and after loading a model I can then right click on it and add an STL to it. Then it’s a matter of positioning and lining them up. Once done you can slice it as one model.
I’m using a resin printer so Orca won’t work for me.
I believe you can export as an STL from Orca Slicer, though I haven’t done it myself. You’d basically merge the STLs in Orca, export, and then open the exported STL in the slicer of your choice.