What does Frostbite do

Two-handing a weapon in DS3 works a bit differently than in previous games.

The most important change is the addition of Weapon Arts, which, depending on the weapon, can do a great deal of damage, counter, or give versatility to your moveset. In any of these cases, your offensive potential is augmented.

Two-handing a weapon also can allow you to properly equip a weapon you do not meet the strength requirement for, as two-handing will add 50% of your strength towards meeting the requirement.

Lastly, for damage output, there is an increase, but it varies from weapon to weapon.

See this Reddit thread:

Name (Strength Scaling): 1H - 2H (% Increase)
Mail Breaker (E): 32 - 36 (12.5%)
Pontiff Knight Curved Sword (D): 119 - 138 (16.0%)
Hand Axe (D): 99 - 121 (22.2%)
Astora Greatsword (D): 128 - 157 (22.6%)
Irithyll Straight Sword (D): 120 - 145 (20.8%)
Longsword (D): 102 - 126 (23.5%)
Heavy Longsword (C): 107 - 134 (25.2%)
Sharp Longsword (D): 111 - 130 (17.1%)
Raw Longsword (-): 96 - 101 (5.2%)
Broadsword (C): 112 - 137 (22.3%)
Great Club (C): 147 - 185 (25.8%)
Club (C): 90 -119 (32.2%)
Heavy Club: 110 - 140 (27.3%)
Reinforced Club (C): 94 - 123 (30.8%)
Butcher Knife (A): 90 - 127 (41.1%)
Deep Battle Axe (-): 104 - 111 (6.7%)

Have you activated the bonfire after the barrier yet? As far as I know you have to activate that bonfire in order to make her sign appear on the bridge. It's on the far end of the bridge - where you came from the Catacombs.