Combining two A2 plots onto a A1 sheet

you could insert one (or xref) into the other & do that way

In AutoCAD you are not limited to what you can plot, nor the size of the final sheet. The latter is a limitation of your printer, and nothing that I know of limits the first.

If you came to acad after they started calling paperspace "layouts", there is a bias in the program you know that tends to push you in the direction of "one sheet per layout", but you are not stuck with this. If you put two title blocks in the same layout, with viewports within each, all you have to do is set the thing up to plot both. Before "layouts" this would have been called plotting by window or view.

put them all together and plot by window. as stated before, you ar only limited by the plotter capabilities