It’s the same entrance and entry desk. Show the lounge agent both your BPs and you will be directed to the appropriate lounge. There’s not a great a deal of difference to be fair, and the 1903 is being retired in the next few months. Ignore any entry signs that indicate the lounges are at capacity, those signs are intended for walk-up Priority Pass (and similar) customers who have not pre-booked. Airline guests are on the ‘guest list’ and will be admitted notwithstanding the notice.