I have Suitelet that displays a custom record which is the head of an order. To this custom record there’s a second custom record that holds the item lines (one for each line). In this Suitelet that I have, I want a link on each row that should open up a window displaying its item lines. How is this best approached?

I'm assuming that your item lines are displayed in a sublist that you populate on the Suitelet. When you populate that sublist, you just need to make the first column a hyperlink to the url of that item line custom record. And then depending on how fancy you want to get you can have it open in a new tab using window.open on your client script for that Suitelet.

Basically I need two Suitelets? Was curious if it was possible to do in one