I am creating svg icons for home assistant. Many are duotone. The requirement is for one colour to be path fa-primary and the other fa-secondary. I have viewed some duotone icons in Home Assistant and some examples have the individual paths defining areas that do not touch. An example is below where I am only showing one path (fa-primary).

When creating my own, my duotone paths for black and grey are split as a result of the space between. I have temporarily named them black1/2 and grey1/2 o make it clearer. 

How can I combine the black into one path and the grey into one path? and it can only be done in other software? I assume this must be possible as the working svg is clearly one path for the grey. Unless there is a limitation in Inkscape and it can only be done via other software?

You have two clips on grey2 and black2 you need to remove.

1. Select grey2, right clic > clip > release : this leaves you two paths selected : unclipped grey2 and its clipping path : you'll do a boolean intersection operation on them : with them still both selected do path > intersection (ctrl+*)
2. Repeat for black2

Then select black1 and black2 and do path > combine (ctrl+k). Redo for grey1 and grey2

It also seems that you create the grey by giving to grey1 and gray2 a partial opacity (87% ?) : I guess its intentional.

Thanks. Actually, when I followed your instructions, " path > intersection" created the path behind the right object, which I guess makes sense. When I changed this to "difference", it worked. I did not understand about clipped paths behind so you have taught me something new. Cheers