It turns out there is a method I was missing here ... found indirectly via https://github.com/mongoid/mongoid/issues/2845
Given these two queryables:
a=Person.where({'name.first_name'=> /a/i})
b=Person.where({'name.first_name'=> /j/i})
You can combine them using .selector
Person.or(a.selector, b.selector).to_a
=> selector={"$or"=>[{"name.first_name"=>/a/i}, {"name.first_name"=>/j/i}]}
or
Person.and(a.selector, b.selector).to_a
=> selector={"$and"=>[{"name.first_name"=>/a/i}, {"name.first_name"=>/j/i}]}