This mode is save compatible, and work only with single player.
You need Harmony

The principle is simple :
You go front, the back party follow the front party.
You go back, Use Ctrl+Click to drive the front party.

Need a companion or an other hero in your party before split

Shortcut : Ctrl + X
(X can be change in config.json file)

All you take with one of your party are merged when you merge again.

This functionality (Ctrl+X) can be using from an army too ;)

Since V2 You can use one of your partie in your army like a hunting dog
through dialogs.
Use Ctrl+Click to drive the hunting dog partie
talk to the party leader to merge the party in your army back.

Since 2.0.4 The Hunting dog no more have the disorganized malus
and have the same speed bonus of the player (if you select it at the start of the game)

Mod who go well with SplitParty :

Usefull Companions
True relation
Party Screen enhancement
Marry anyone
... what you want no conflict found actually

Version 2.0.11
Fix a bug SplitParty chrash M&B 1.7.2 on Load
Fix a bug : You can't no more start a split without at least 2 Hero
in your team

Version 2.0.10
Fix a bug in the Fix of 2.08

Version 2.0.9
Compatible with M&B 1.7.2

Version 2.0.8
Patch : If you atk the Conspiracy army with your front and back party, you
could have Conspiracy prisoners.

Fix :  If the party was not in army before split
(player join front) and the backParty is enroled in army.
The party leave the army when they join again

Version 2.0.7
Fix a bug Leader of the backParty join the frontParty
because the save of the troopRoster
=> The merge party don't work well and your lost
the loot (prisonner and item) from the front party
Fix a bug, after loading the game with split active and mainHero in front
there was an endless loop if you click on the map with Ctrl

Patch : Hero leave the other party of the mainhero party when loading
the game with split mode actif Following an Update of M&B 1.7

Version 2.0.6
Fix the hunting dog booster speed wich was too strong

Version 2.0.4
SplitParty v2 For MB 1.7.0 version 2.0.4
Cheating : The Hunting dog start very fast now, he doesn't have the disorganized malus
and if the player have take a bonus at start of the game, the hunting dog have it too.
XML are updated (for traduction)

Version 2.0.3
Fix an technical issue : with MB 1.7.0 and ImprovedGarison, the BackParty without MainHero
can lost deseter because the size limit was not well patched in this condition

Version 2.0.2
Fix a bug (see in buglist) on MB 1.7.0.
Fix a late start (1 hour) of the vanguard when player is back on MB 1.7.0.
Fix a hearing fail of the vanguard if he enguaging a party and you want to swap
the directive, on MB 1.7.0.

Version 2.0.1
You can ask one of your parties leader (in your army) to
play the hunting dog (like you can make with your vanguard when you are in backgard)

Version 1.0.10
Vanguard hear your orders better (When you are in the backguard)
but AI stay active

Version 1.0.9
Vanguard is more Obediant when you are in the backguard
config.json in C:\Users\'User'\Documents\Mount and Blade II Bannerlord\Configs\ModSettings\SplitParty now
Fix : Hero no more mixe up in the party

Version 1.0.8
Patch Hero twins or + (TOO much of the same hero)
If your units count in your party dismatch your party count on the map
Save and reload => the number will be back good

Version 1.0.5
The other splited sparty don't recrut more units from the start of the split
The merge roster work well (the units order is the same as before the split)

Version 1.0.4
a little cheat, if your other party can keep less units then you, but in fact she can keep the same number ;)