3D Studio Max
Insert a vertex, then make visible the edges in red and the dotted red one invisible. (high right).
Lastly, with Edge Cut , insert a division into the model by activating the 3DSnap MidPoint.
Add Meshsmooth to check out your fantastic bit of modelling :)
Possibly make any necessary vertex adjustments.
Here the smoothing is not perfect so I added a vertex and arranged the edges.
Make invisible the edges shown in dotted lines.
As usual, do not make vertex adjustments in the meshsmooth level
They are not necessary as you will see when joining the two halves of the armour.
Extrude as on the left
Then angle the Faces with Rotate on Y.