Think this will work:

* item 1
 ** item a +
    paragraph 1 inside item a
+
--
*** item a.1 inside paragraph 1 inside item a +
    paragraph a.1 inside item a.1
*** item a.2 inside paragraph 1 inside item a +
    paragraph a.2 inside item a.1
--
+
paragraph 2 inside item a
 ** item b +
    paragraph 1 inside item b
* item 2

Tested with Asciidoctor 1.5.6.1.