I believe I've managed to find it, it's called 'Yvain et Yvon' in French, and 'Isegrims Abenteuer' in German (not sure about the Dutch title), and was written by Patrick Cadot.
It was published in Tintin/Kuifje magazine in Belgium, which is probably where you found it (and where I remembered it from), though it was also published by Yps in Germany, which is the one place I've found that actually has information on it.
Not sure how to provide excerpts in this case, but I'll try linking images, so you can see if it's familiar to what you've read:
![Jonas and Isengrim, beneath a tree on a high river bank, look out at a town on the other side of the river](https://cdn.statically.io/img/www.ypsfanpage.de/comics/isegrim/ise3.gif)
![Jonas hugging Isengrim](https://cdn.statically.io/img/www.ypsfanpage.de/comics/isegrim/ise5.gif)
Some quotes, on DavidW's recommendation. Still in German, since doing a fully correct English translation is a bit too difficult for me:
Die pfiffigen Zwillinge Jonas und Juri sind alles andere als normale Kinder, denn Juri besitzt die Fähigkeit sich in einen Wolf zu verwandeln. Als Wolf Isegrim besitzt er magische Fähigkeiten...
[The bright twins Jonas and Juri are normal in every way except that Juri has the ability to turn into a wolf. As the wolf Isengrim he has magical abilities...]
rough translation by DavidW; errors mine
Juri und Jonas sind zwei ganz normale pfiffige und liebe Zwillinge, die sich gut verstehen. Sie sind abenteuerlustig und gelangen dadurch manchmal sogar in gefährliche Situationen. Juris unglaubliche Geheimnis teilen die Zwillinge nur mit ihren besten Freunden und nur, wenn es wirklich sein muss. Selbst ihre Eltern wissen nichts davon.
[Juri and Jonas are two completely normal, bright, and loving twins in synch with one another. They are eager for adventure and sometimes end up in dangerous situations. The twins only share Juri's unbelievable secret with their best friends, and only when they truly need to. Even their parents don't know.]
rough translation by DavidW; errors mine
Isegrim, dem Wolf mit den glühend roten Augen, umgibt ein Geheimnis: Er ist immer da, wenn Juri verschwunden ist: Isegrim ist Juri und Juri ist Isegrim!
[Isengrim, the wolf with glowing red eyes, has a secret: He's always there when Juri disappears: Isengrim is Juri and Juri is Isengrim!]
rough translation by DavidW; errors mine
Edit 2: Found the Dutch title: 'Ed en Ad'