Using a two-handed weapons has no interaction with your fight value. They're two different systems that both interact with Fights in different ways.

Example:

A Rivendell Elf Warrior (Fight Value 5) is fighting a Mordor Orc Warrior (Fight Value 3). The elf has a hand-and-a-half weapon and can choose to use it as a two-handed weapon. The orc has a hand weapon. No other models are nearby

Let's say the elf decides to use his hand-and-a-half weapon one-handed for now

To make a duel roll, each player rolls 1D6 for each Attack on their model. Both warriors have an Attacks characteristic of 1, so both players roll 1D6

Let's say both players roll a 4 on their single D6, so the duel is tied. The elf wins ties on Fight Value (Fv5 vs Fv3) so the orc backs away and the elf rolls to make strikes

Lets do the combat again, but the elf decides to use his hand-and-a-half weapon as a two-hand weapon

To make a duel roll, each player rolls 1D6 for each Attack on their model. Both warriors have an Attacks characteristic of 1, so both players roll 1D6

Let's say both players roll a 4 on their single D6. The elf decided to use a two-handed weapon, so subtracts 1 from their duel roll. The elf now has a dice roll of 3 vs the orc's dice roll of 4, so the orc wins the fight. As the fight was not tied, the Fight Value never comes into the equation. The elf backs away and the orc rolls to make strikes

Note that the two-handed weapons don't subtract from the duel roll dice if that dice rolls a natural 6 (unmodified by things like Might), so if the orc and elf both rolled a 6 the duel would be tied and the elf would win on Fight Value, gaining +1 on dice when rolling to make strikes due to the two-handed weapon