Hello! Quick question:
Is it possible for a fighter to keep an enemy grappled while attacking with Dual-Handed Assault?
Probably not RAW. But I would personally allow it as a houserule.
The rules problem is with the requirements of using Dual-Handed Assault
Requirements: You are wielding a one-handed melee weapon and have a free hand.
Also, maintaining a grapple isn't on the list of things that Dual-Handed Assault expressly allows.
This action doesn't end any stance or fighter feat effect that requires you to have one hand free.
Since grapple isn't a stance or fighter feat.
For characters that only have two hands: if one is holding a weapon and another is holding an enemy (currently in a grapple), then you don't have a hand free any more.
If you somehow have a third hand or other appendage that can maintain the grapple (Syu Tak-Nwa's Deadly Hair for example), then you could be holding a weapon in one hand, the enemy in the special grapple hand, and have a hand free for Dual-Handed Assault. But at that point, you could just change grip on your weapon to two-handed.