So the Armor Master has Deflective Shield. It basically reads as a scaling bonus to touch AC gated by total shield bonus.
At 2nd level, an armor master specializes in using his shield to deflect attacks. He gains a +1 bonus to his touch AC, and this bonus increases for every four levels beyond 2nd (to a maximum of +6 at 20th level); however, this bonus cannot exceed the sum of the armor and enhancement bonus to AC provided by the shield that the armor master is currently carrying.
This ability replaces bravery.
Then there is Mobile Fortress, which gives you half of the total shield bonus to AC to touch.
Prerequisite(s): Str 15, Mobile Bulwark Style, Shield Focus, Tower Shield Proficiency, base attack bonus +4.
Benefit(s): While using a tower shield, you add half of the shield’s bonus to AC (including its enhancement bonus) to your touch AC and take only a –1 penalty on attack rolls because of the shield’s encumbrance. While using Mobile Bulwark Style, whenever you would use a tower shield to gain total cover, you also grant any adjacent ally total cover against attacks passing through the selected edge of your space.
It seems like this allows for a full (tower) shield bonus to touch. Am I reading this right?
P.S. The Deflective Shield ability does have a typo. It should scale up to +5.
Nothing?
They stack, fully. Deflective Shield adds an untyped bonus to your Touch AC, Mobile Fortress adds half your Shield AC to your Touch AC.
Not only does this allow you to add your tower shield AC to touch, it actually allows you to exceed it at high enough levels; Deflective Shield doesn't count the AC from Mobile Fortress against itself.