Although generally not allowed in tournaments, in home games fielding two detachments can be a legitimate and effective strategy in larger games, especially in how special characters day by day are defining armies.The kick is your opponent must agree to allowing you to use multiple detachments. A good sport should let you in games held just for fun.
The Requirements of Two Detachments:
1 HQ
2 Troops
1 HQ
2 Troops
Now yes, this may force you to take some more troops than you normally would want to field, but it allows you several other advantages.
1-If your army has a 0-1 choice on a piece of wargear or unit choice, then you are now allowed two of them, as there are now two armies, as described on page 87 of the rulebook under multiple detachments. Just put one in each detachment.
2-If a character’s rules get overturned by another characters rules, say like Chapter Tactics, then you may use both sets of rules on one half of the army each (like if you have Crimson Fists fighting along side White Scars, Kantor leads one, Khan the other).
3-Rules that affect “friendly units” have an effect on both detachments. Men in one army would be just as inspired by something of great import as men in the other army.
Disadvantages/Limitations.
1-You have to take 4 troops and 2 HQ compulsory
2-You may not take 2 of the same character
So with these point in mind here is an example 3000 pt Space Marine list:
Det 1- Crimson Fists
HQ- Pedro Kantor- 175
Troops- Tactical Squad(10)- 230: Melta Gun, Plasma Cannon, Power Weapon, Rhino
Troops- Tactical Squad(10)- 235: Melta Gun, Lascannon, Power Weapon, Rhino
Elites- Sternguard(5)- 205: 5 x Combi Meltas, Razorback w/ Extra Armour
Elites- Sternguard(6)- 230: 5 x Combi Meltas, Razorback w/ Extra Armour
Heavy Support- Devastator Squad(5)- 210: 2 x Lascannon, 2 x Plasma Cannon
Heavy Support- Predator- 110: Heavy Bolter Sponsons, Extra Armour
Heavy Support- Whirlwind- 85
Pts: 1480 Scoring Units: 4-6 KPs: 12, 15 (Hard Boyz)
Det 2- White Scars
HQ- Khan- 205: Moondrakken
HQ- Chaplain- 130: Terminator Armour, Storm Bolter
Troops- Bikes(5)- 185: Powerfist, 2 x Meltaguns
Troops- Bikes(5)- 165: Power Weapon, 2 x Flamers
Elites- Terminator Assault Squad(7)- 545: TH & SS, Land Raider Crusader, Extra Armour
Fast Attack- Assault Squad(5)- 160: 3 x Plasma Pistols, Power Weapon
Fast Attack- Scout Bike Squad(5)- 130: 2 x Astartes Grenade Launchers
Pts: 1520 Scoring Units: 2 KPs: 8, 14 (Hard Boyz)
Total
Pts: 3000 Scoring Units: 6-8 KPs: 20, 29 (Hard Boyz)
So the Crimson Fists provide a solid gun line while the White Scars Outflank your opponent entirely and hit hard from behind. Khan goes with some bikers to make them even beastlier. Terminator Chaplain with the Terminators to give them Liturgies of Battle and Honour of the chapter (re-roll failed to hits and make them fearless).
In the first list, Kantor sticks with the smaller Sternguard unit. Grab objectives and sit and shoot. If objectives are out of reach, transports or WS bikes can make it.
Part two will be two detachments with a twist.