The airline unveiled the deal, which includes flights from London Stansted and Manchester airports, to mark its 30th anniversary.
Passengers travelling from Stansted will receive two adult tickets for the price of one on routes to Düsseldorf, Hannover, Münster, Nuremberg and Paderborn.
Travellers heading from Manchester Airport to Paderborn, in the North Rhine Westphalia, can also take advantage of the special offer.
Travel May 1-31. Individual bookings are limited to a maximum of four passengers. All fares are subject to availability.
At least one passenger in every group must be a full-fare payer, and children must book under the adult fare in order to qualify for the 2-for-1 offer.