Two RSS feeds, different items limit?

Sure, you can. First create the feed as per codex:

function anotherfeed_init(){
    add_feed('anotherfeed');
}
add_action('init', 'anotherfeed_init');

Now, change the post count for this particular feed:

function anotherfeed_post_count( $query ) {
    if( $query->is_main_query() && is_feed('anotherfeed') )
        $query->set( 'posts_per_rss', 30 );
}
add_action( 'pre_get_posts', 'anotherfeed_post_count' );