If both companies are having the same production line, what kind of merger they can do? Horizontal or vertical?

As they have the same production line so there no supplyer and customer relationship to be a vertical merger.

Simply it is only a Horizontal merger in the same industry and same product.

The goal of merger or acquisition is to earn more profit, less their expenses, make a larger market all around the world and be more popular which actually benefit for both sides of the business.

And the same product which both the companies are having can still (should) run in market Horizontally because …products are need / choice bases…