Looks like from various logs, you can stack stoneform on top of an existing corporeal hardening spell, but the way it's done, it looks like you aren't able to cast corporeal hardening when affected by stoneform. Given stoneform's similiarity to stone skin, which corporeal hardening won't stack with, I figured I'd ask.

<1191/1422(83%)hp 88/1091(8%)m 98%mv new 9 AM> nap

The Nightwalker checks the ground for tracks.
You calm as your stream of adrenaline tapers off.
*** ADRENAL BURST DOWN! ***
You feel less resistant to mental attacks.
*** RESIST MENTAL DOWN! ***
Your skin loses its magical firmness.
*** CORPOREAL HARDENING DOWN! ***

<1237/1422(86%)hp 451/1091(41%)m 100%mv new 10 AM> You calm your adrenaline

and take a nap.

<1237/1422(86%)hp 451/1091(41%)m 100%mv new 10 AM> stand
You wake and stand up.

<1237/1422(86%)hp 451/1091(41%)m 100%mv new 10 AM> rem stoneform
c 'corporeal hard'
wear stoneform
You feel utterly drained as you remove the amulet and change back to normal.
You stop using a stoneform amulet.

<1237/1392(88%)hp 451/1091(41%)m 0%mv new 10 AM> Your flesh grows tougher

and more firm.

<1237/1392(88%)hp 411/1091(37%)m 0%mv new 10 AM> sleep
You wear a stoneform amulet around your neck.
Your joints seize up as your flesh is transformed into stone!

<1237/1422(86%)hp 411/1091(37%)m 0%mv new 10 AM> You go to sleep.

n/t