[cybmole]

surely    inserts 2 spaces.

I've seen indents done that was, in coversions & they work.

in code view
<p>sentence.&nbsp;&nbsp;nextsentence.&nbsp;&nbsp;l ast sentence</p> should work..
yep works as expected.

now if you want to force that throughout an existing book we;ll have to get into using find/replace.

a find/replace option could detect a full stop followed by a single space followed by a Capital letter, and repalce that with full stop , 2 x nbsp, folloed by the original capital letter.

something like
find
\. ([A_Z])
replace
.&nbsp;&nbsp;\1

find replace options: match case, regex, all files, replace all..

you should be able to type as you go, in book view, but it seems bugged.

I get one space left in , for every two spacebar presses. so you can type in multiple spaces, just have to type twice as many as you really want.
e.g. in a new sigil document I generated this html by pressing space bar 4 times between each word as I typed the line in book view

<p>here &nbsp; &nbsp;is &nbsp; &nbsp;a &nbsp; &nbsp;new &nbsp; &nbsp;test.</p>

that was with tidy unticked. same result with it ticked:

<p>and &nbsp; &nbsp;one &nbsp; &nbsp;more &nbsp; &nbsp;with &nbsp; &nbsp;tidy &nbsp; &nbsp;on.</p>

save the test file, exit sigil, restart sigil load the test file. All is good.