I have two polygons that consist of two separate polygons/lines in QGIS). How can I make this into a multipart polygon, which consists of 1 line in QGIS?
For anyone coming to this with newer versions of QGIS, the function to do it is now called "Collect Geometries" rather than "Single parts to Multipart" so it's now Vector > Geometry Tools > Collect Geometries. You can then choose a field (or fields) to use as your grouping variable (Unique ID Fields). I used it to fix up some statistical areas which included non-contiguous polygons (islands in fact) where I'd ended up with 1 record for each separate polygon, but wanted 1 record for each statistical area. The reason I needed THAT was because I'm joining it to data in Tableau and ended up with a whole bunch of duplication happening around the coast due to the islands.
Vector->Geometry Tools->Single parts to Multipart
However, you say that you only want a multipart polygon consisting of a single line. You are unlikely to get that ever because that would be a single polygon, because, by definition, any multipart geometry has multiple parts.
You could also dissolve your data and that will create a multipart if you dissolve polygons which do not intersect but will also combine polygons which do intersect into single units and perhaps this is what you meant (Vector->Geoprocessing Tools->Dissolve)