User user = userService.getUser (userId);
UserPreferences prefs = userPreferencesService.getPreferences(userId);

CompletableFuture<User> userAsync = CompletableFuture.supplyAsync(() -> userService.getUser (userId));
CompletableFuture<UserPreferences> userPreferencesAsync = CompletableFuture.supplyAsync(() -> userPreferencesService);
CompletableFuture<Void> bothFutures = CompletableFuture.allOf(userAsync,userPreferencesAsyn);
bothFutures.join();
User user = userAsync.join();
UserPreferences prefs = userPreferencesAsync.join();

@GetMapping(" /users/{userId}")
public Mono<User> getUserDetails(@PathVariable String userId) {
    return userService.getUser(userId).zipWith(userPreferencesService.getPreferences(userId))
    .map (tuple -> {
        User user = tuple.getT1();
        user.setUserPreferences(tuple.getT2());
        return user;
    });
}