A small guide I made for one to another mod but it works the same way 
-----------------------------
Open unit_models\battle_models.modeldb
find the unit you want to add, you can also look what the unit is named in Export_descr_unit
for example:
armour_ug_models Heavy_uruk_sword
search for it in battle_models.modeldb
look for:
Code:
16 Heavy_uruk_sword
1 3
66 unit_models/_Units/ES_Greek_Greek_Heavy/Heavy_Uruk_Sword_lod0.mesh 121
66 unit_models/_Units/ES_Greek_Greek_Heavy/Heavy_Uruk_Sword_lod0.mesh 1225
66 unit_models/_Units/ES_Greek_Greek_Heavy/Heavy_Uruk_Sword_lod0.mesh 6400
1
6 france
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_IS_Diff.texture
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_IS_Norm.texture
47 unit_sprites/france_Heavy_uruk_sword_sprite.spr
1
6 france
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_weapons.texture
75 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_weapons_norm.texture 0
and make it look like
Code:
16 Heavy_uruk_sword
1 3
66 unit_models/_Units/ES_Greek_Greek_Heavy/Heavy_Uruk_Sword_lod0.mesh 121
66 unit_models/_Units/ES_Greek_Greek_Heavy/Heavy_Uruk_Sword_lod0.mesh 1225
66 unit_models/_Units/ES_Greek_Greek_Heavy/Heavy_Uruk_Sword_lod0.mesh 6400
2
6 france
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_IS_Diff.texture
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_IS_Norm.texture
47 unit_sprites/france_Heavy_uruk_sword_sprite.spr
7 england
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_IS_Diff.texture
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_IS_Norm.texture
47 unit_sprites/france_Heavy_uruk_sword_sprite.spr
2
6 france
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_weapons.texture
75 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_weapons_norm.texture 0
7 england
70 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_weapons.texture
75 unit_models/_units/ES_Greek_Greek_Heavy/textures/Heavy_weapons_norm.texture 0
To make it more understandable, the values before all lines are the count of letters and stuff.
And the value I increased from 1 to 2, is the count of how many factions that
are using the model textured.
And don't leave any empty spaces, it will make the file unreadable!
Then jump back to Export_descr_unit.txt
and go to the unit you was adding
And after all lines that ends with france add , england
simply this:
Code:
ownership france
era 0 france
era 1 france
era 2 france
to this
Code:
ownership france, england
era 0 france, england
era 1 france, england
era 2 france, england
You said you had some rusty experience, so I made it fast.
I hope that it will help. 
also if you want the unit recruitable on the campaign map,
go to export_descr_buildings
and search for the unit
and then this:
Code:
recruit_pool "Uruk-Hai Infantry" 1 0.5 4 0 requires factions { france, } and region_religion catholic 40
to this:
Code:
recruit_pool "Uruk-Hai Infantry" 1 0.5 4 0 requires factions { france, england, } and region_religion catholic 40
I also have to add, that I strongly recommend this tool.
http://notepad-plus.sourceforge.net/uk/site.htm
When you have learned how to fully master it you can add plenty of units for other factions
in a couple of minutes. (=
A screen that shows how to count letters using it: