reproduce https://github.com/sferik/rails_admin/issues/2626

console

$ rails -v
Rails 4.2.6
$ rails new nested_update
...
$ cd nested_update
$ echo "gem 'rails_admin', '~> 0.8.1'" >> Gemfile
$ bundle
...
$ rails g rails_admin:install
...
<enter>
...
$ rails g model person name
...
$ rails g model cat person:references name
...
$ rails g model dog person:references name
...
$ rake db:migrate
...
$ vi app/models/person.rb [see file below]
$ rails s
...
$ open http://localhost:3000/admin/person/new [in a new tab]

person.rb

class Person < ActiveRecord::Base
  has_one :cat
  has_one :dog
  accepts_nested_attributes_for :cat, update_only: true
  accepts_nested_attributes_for :dog, update_only: false
end