I tried it right now with my own profile and yours, and it works. (Of course, I did not go on requesting to merge your profile with mine.) Log in to https://www.researchgate.net/profile/Daan-Bronkhorst-2 (I guess this is the profile you want to keep as you wrote this question from this profile, and you will already be logged in when you read my answer). Then open https://www.researchgate.net/profile/Daan-Bronkhorst and click on "More" (at the right-hand side, right from "Follow" and "Message"), then click on "Report duplicate" and type in the corresponding email addresses.
Dear Wolfgang Dick, somehow I did not find the right button and now, following your guide precisely, it turned out not to be a problem to merge the two accounts. Also I can now add full texts where they are still missing.