Open a new blender project. From the drop down menus along the top, Select open, import, import STL, then find your first stl in the file explorer. Repeat this to import the second stl. Drag and rotate both objects until they're lined up how you want them. Select both stls at the same time. Right click and select "merge". Then in the drop down menus, find export, export as STL. Save it as your new STL. Open this new stl in your preferred slicer program, and you're good to go!
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.
if the file is small enough you can throw it in tinkercad and merge it.
Cura will let you print it as one file if you let it but you have to set it correctly.
Before you mess with extra software try dragging all parts to the slice at one time. It should ask if they are parts of one object (orca does this) then select yes. They should all go into the right place, hopefully