Apologies for the multiple posts - my PC developed a stutter just as I was posting.
If the four shaft pattern calls for a 10 dent reed and there are two threads per dent, then you will have 20 ends per inch. It would be the same on the rigid heddle loom, you would use 2 heddle reeds the same size, 10. This is a good sett for threads like cottolin or 8/2 cotton when weaving plain weave or a pattern weave with a plain weave ground weave.
From the chart for the 4-shaft pattern, there is one thread per shaft. I'm not familiar with floor looms - does one thread per shaft equal two threads per dent?
There is no relationship between threads per heddle and threads per dent. Almost all weave structures use one thread in each heddle. How many threads in each dent depends on how dense you want your fabric. When I thread a floor loom with 8/2 cotton for twill towels, I put one thread in each heddle (the heddle is the wire with the eye in it that the yarn is threaded through; the shaft is the frame that holds the heddles), and two threads per dent in a 12 dent/inch reed. In your RH loom, the heddle serves both to lift the threads to form a shed, and beat the new weft to the fell, and you control the density of the fabric by the heddle you choose . In floor and table looms, the shafts lift the threads and the beater and reed beats the new weft to the fell, The density of the fabric is controlled by how you thread (sley) the reed. The 24 epi that I use for my twill towels give me a firm, soft fabric. 36 epi (three threads per dent) would give me a very hard, stiff fabric. 12 epi (one thread per dent) would give me a loose, gauzy fabric. None of these changes in sett would affect the threading in the heddles.