I'm new to baan4, i need to create a header-line type of form, with 2 tables and how can i achieve that

Is this just a display session? That will make a big difference. So if it is a modify session search this board for combining sessions. This is not exactly easy and is not something I would recommend for someone new. If new tables I would recommend a session to modify the header and a button on this form to launch another session to modify the lines.

So assmuing this is just a display session you use the generate session in baan tools. You generate a multi-occurence session on the lines table. So for example you could generate the session on purchase order lines. You would want only the Purchase order as the view field. Then after generating the session you can modify it to add display view fields for the header. Then in the after.display even for the purchase order you can do a quick query for the header info you want to display.