The reason why there isn't much information about this ... is because, other than a Tutorial example (see the Tutorial Section) about a Torpedo that can sometimes knock down Shields and do hull damage... but is a bit buggy... there is no way to do this, without using some fakery.

Can you do it with fakery? YES!!

Make a gun (or modify an existing model), and add two HpFire points. Put them at the same place.

Now you have a gun that will shoot two shots at the same time, both aligned, etc., exactly the same. Now you have a gun that can (theoretically, but online this probably won't work right due to the way lag compensation works) hit a Shield... and if the first of the two hits knocks down the Shield, the second shot will go right through and hit the hull. The only tricky bit with multiple HpFire points is that you need to assign the sound effect differently than normal. This is covered in a Tutorial.

The other method of fakery is to use my True Beam Weapon technique... but that's going to have to wait until Toolkit 1.3 is released- I'm keeping that concept under wraps until then

But basically... other than the Torpedo of Doom, which either works right... or does no damage at all... the only way to accomplish these things is through fakery. There's no way built into the game engine's core behavior (that we know of, at any rate) to get around this.

what about the explosion lifetimes?surely one is energy and the other is hull and if you make hull damage longer lasting than energy it will still be around to kill the ship after the energy damage does its work

<shrugs> I'm not sure that that's what those numbers mean, but if so, that'd make for an interesting development. Want to try a test case? If it works, I'll be happy to throw a "dual-damage torpedo" example into Toolkit

YES, it has been confirmed! By fiddling with the lifetimes of an explosion, you can make a shielded ship go *kaboom* in one shot!

Here, I'll post the code, which I modified the Sunslayer(since that's what my Defender had at the time).

[Motor
nickname = torpedo01_mark02_motor
lifetime = 11.250000
accel = 3.555600
delay = 0

[Explosion
nickname = torpedo01_mark02_explosion
effect = ku_torpedo01_impact
lifetime = 0.000000, 1.000000
process = disappear
strength = 100
radius = 25
hull_damage = 11736
energy_damage = 11000
impulse = 0

[Munition
nickname = torpedo01_mark02_ammo
explosion_arch = torpedo01_mark02_explosion
loot_appearance = ammo_crate
units_per_container = 1
hp_type = hp_torpedo
requires_ammo = true
hit_pts = 2
one_shot_sound = fire_torpedo
detonation_dist = 6.250000
lifetime = 56.250000
Motor = torpedo01_mark02_motor
force_gun_ori = false
const_effect = ku_torpedo01_drive
HP_trail_parent = HPExhaust
seeker = LOCK
time_to_lock = 0
seeker_range = 2700
seeker_fov_deg = 35
max_angular_velocity = 0.580670
DA_archetype = equipment\models\torpedoes\ge_torpedo.3db
material_library = equipment\models\ge_equip.mat
ids_name = 265164
ids_info = 266164
mass = 1
volume = 0.000000
hull_damage = 11000.600000
energy_damage = 11000

[Gun
nickname = torpedo01_mark02
ids_name = 263164
ids_info = 264164
DA_archetype = equipment\models\torpedoes\ge_torpedo.3db
material_library = equipment\models\ge_equip.mat
HP_child = HPConnect
hit_pts = 6752
explosion_resistance = 1.000000
debris_type = debris_normal
parent_impulse = 20
child_impulse = 80
volume = 0.000000
mass = 10
hp_gun_type = hp_torpedo_special_1
damage_per_fire = 0
power_usage = 0
refire_delay = 2
muzzle_velocity = 20
toughness = 58.700001
projectile_archetype = torpedo01_mark02_ammo
dry_fire_sound = fire_dry
separation_explosion = sever_debris
auto_turret = false
turn_rate = 90
lootable = true
LODranges = 0, 20, 60, 100

Very cool- I'll playtest it and see where I can fit that idea into the game design