Thalys has a special “Happy Hour” sale good until Friday, 23 April. You pay for one ticket and you get the second one (same train) for your travel companion for free. That means you pay 19 euros each way for two people in 2nd class (29 euros in 1st class).
This offer is good for travel from 14 June to 16 July. All the details are on the Thalys page.