I find that double depth elements are very time consuming to base, because it is very hard to get the brush or palette knife into the space between the two ranks of figures. Moreover, sometimes paint or gunk gets onto the finished figures which then need retouching. Yet as the number of miniatures I own increases, I find that I want to base more and more miniatures this way, because they are quicker to deploy and move on the table.
Above are some of the splendid Foundry/Black Tree German figures that Nick has painted for me. I've done very little too them aside from painting the bases to match the eventual colour of the earth that will surround them, and to varnish them. I've arranged them on six 60x30 magnetic bases of my own manufacture.
In the above closeup, you can see that I like to position some of the bases so that the figures straddle both bases; this looks natural, and has practical value at a later stage. I also like to use irregular numbers of miniatures, when possible. The miniatures are stuck in place with wood glue, which dries relatively slowly and gives one time to move them around to get the most attractive arrangement.
Next, place the two bases next to each other on a magnetic surface, and cover each with your basing compound; I use a mix I've made up, that I call BigRedBatGunk (n.b. there is a square of tinplate under the bases in the photo). Then add a little extra gunk along the top of the seam to disguise it. It will then appear something like the above; the magnets hold the bases in place whilst the gunk dries like concrete.
And, once the gunk has dried, the two bases can be snapped apart, as above, leaving an irregular line along the break. I then paint the two half-elements, separately. Once fully painted, the two bases can be stuck back together along the edge with superglue, and this rough join is very much harder to see than a straight line would be; in fact virtually invisible. Any figures straddling the join (as above) help to make it stronger. I'll show the finished bases in a later post.