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!
I use an old Britta filter with some cheese cloth to filter my old IPA. Pour in the old nasty stuff, let it sit for a day, pretty clean reusable IPA comes out the other end.