routes.rb

devise_for :users, skip: :all
devise_scope: use do
  get 'admin/login', to: 'amin/sessions#new'
end

isso funciona?

devise_scope :user do
  get "/user/login", to: 'devise/sessions#new'
  get "/admin/login", to: 'devise/sessions#new'
end

Desse jeito funciona, testei aqui, mas acho que talvez seja melhor ter dois models mesmo por que ai voce não vai ficar se preocupando com permissões de acesso 🤔

Rails.application.routes.draw do
  devise_for :users, path_names: { sign_in: 'login', sign_out: 'logout' }
  devise_scope :user do
    get '/admin/login', to: 'devise/sessions#new'
  end

  get 'home/index'

  root to: 'home#index'
end

devise_scope :admin do
    get 'admin/login',    to: 'devise/sessions#new', as: :new_admin_session
    post 'admin/login',   to: 'devise/sessions#create', as: :admin_session
    get 'admin/create',   to: 'devise/registrations#new', as: :new_admin_registration
    post 'admin/create',  to: 'devise/registrations#create', as: :admin_registration
  end