Steadfast is in, and detachments don't break steadfast
The enemy will have all his guys in one block and so be steadfast to your two smaller blocks.
I'm not sure if this is true for the following reason. Steadfast requires you to have more ranks than your foe. Now the advantage of detachments is that you are bringing more killing power to the battlefield. A 15 man halberd detachment hitting into the flank of an unit 5 deep is another 10 attacks. Which means that the foe must suffer the attacks that it is suffering from the front and also the attacks that it is suffering from the side (against which it will not be able to use a shield or attack back in two ranks)
Also I think a fifteen man detachment is too small. Just as parent units are getting bigger so too must detachments. We need to move away from the HE position that the 25% is something that is there to be filled so we can move on to our cool stuff. Most of our cool stuff is in our core anyway. If you are not filling your 25% you are doing something wrong. Allow me to look at the maths for a second.
50 halberd (plus full command) 270 points.
20 halberd detachment 100 points.
20 halberd detachment 100 points.
Unit total 470 points.
470 points will give us 90 halberd plus full command. As they are both the same they go together I think. If both parent units are in horde formation and the detachments manage a successful strike on both flanks.
Combat phase one.
50 halberd = 30 attacks, = 15 hits = 10 wounds (none saved) (Working off the hitting on 4/wounding on 2)
20 halberd = 10 attacks, = 5 hits = 3 wounds (none saved)
20 halberd = 10 attacks, = 5 hits = 3 wounds (none saved)
Total 50 attacks 25 hits 16 wounds (none saved)
90 halberd = 30 attacks = 15 hits = 10 wounds (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
Total 38 attacks 19 hits 12 wounds (none saved)
90 Halberds are steadfast and we will assume they pass their L7 roll. Both detachments redeploy to horde formation. We assume only one passes its leadership 7 roll.
Combat phase 2
40 halberd = 30 attacks, = 15 hits = 10 wounds (none saved) (Working off the hitting on 4/wounding on 2)
19 halberd = 19 attacks, = 9.5 hits = 6 wounds (none saved)
19 halberd = 10 attacks, = 5 hits = 3 wounds (none saved)
Total 59 attacks 29.5 hits 19 wounds (none saved)
74 halberd = 30 attacks = 15 hits = 10 wounds (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
Total 38 attacks 19 hits 12 wounds (none saved)
We will assume that they will pass their leadership 7 steadfast roll again. We will also assume that our other detachment managed to pass its leadership 7 reform roll.
Combat phase 3
30 halberd = 30 attacks, = 15 hits = 10 wounds (none saved) (Working off the hitting on 4/wounding on 2)
18 halberd = 18 attacks, = 9 hits = 6 wounds (none saved)
18 halberd = 18 attacks, = 9 hits = 6 wounds (none saved)
Total 66 attacks 33 hits 22 wounds (none saved)
55 halberd = 30 attacks = 15 hits = 10 wounds (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
Total 38 attacks 19 hits 12 wounds (none saved)
Again we will assume that they passed their leadership 7 steadfast roll, which is not certain by any mean at this point. Leading us to.
Combat phase 4
20 halberd = 20 attacks, = 10 hits = 7 wounds (none saved) (Working off the hitting on 4/wounding on 2)
17 halberd = 17 attacks, = 8.5 hits = 5 wounds (none saved)
17 halberd = 17 attacks, = 8.5 hits = 5 wounds (none saved)
Total 56 attacks 28 hits 17 wounds (none saved)
33 halberd = 30 attacks = 15 hits = 10 wounds (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
4 halberd = 4 attacks = 2 hits = 1 wound (none saved)
Total 38 attacks 19 hits 12 wounds (none saved)
Again we will assume that they all pass their steadfast leadership test. At this point both the detachments revert to 5 wide. At least one will manage
Round five
10 halberd = 10 attacks, = 5 hits = 3 wounds (none saved) (Working off the hitting on 4/wounding on 2)
16 halberd = 10 attacks, = 5 hits = 3 wounds (none saved)
17 halberd = 10 attacks, = 5 hits = 3 wounds (none saved)
Total 30 attacks 15 hits 9 wounds (none saved)
16 halberd = 15 attacks = 7 hits = 5 wounds (none saved)
1 halberd = 1 attacks = 1/2 hits = 1 wound (none saved)
Total 16 attacks 7.5 hits 6 wounds (none saved)
Not only this but the 90 has lost its steadfast and therefore is rolling to hold on a -3 all other things being equal.
Disclaimer. I rounded a bit to try and get even numbers. Also I remembered at the end that I had forgotten to include the champion. However what I wanted to make clear, working in a vacuum (which of course none of us are) is that the greater amount of attacks that can be brought to bear by the detachment system means that in a one on one the same number of points (even the same troop type) the detachments system is better. Not once through 5 rounds of combat until it was left with only 8 models did the block of 90 win combat. Not only that but both my detachments are still in reasonable shape, yes my parent unit is destroyed but I am only loosing 270 points and you are loosing 490. Also because of the stupid GW points system as long as the handful of men are still in my parent unit you don't get anything for it.
Just a point to ponder