Let's clear up a few things. Give us a list of the equipment you have relative to this post? I'm confused because you mention Home Cinema, then speak of an Integrated Amp and a Power Amp. Did you misspeak, or am I missing something?
The Arcam A22 is an integrated stereo amp -
Arcam FMJ A22 integrated amplifier | Stereophile.com
I'm not sure of the original selling price but the review above lists it at US$2100 (review date -2003). Power 100w/ch to 8 ohms.
The Arcam P25 is a Stereo Power amp -
P25 Power Amplifier
Also, 100w/ch to 8 ohms. Not sure about the original selling price on this, but I suspect it would be close to that of the A22 Integrated.
The part that confuses me is the reference to Home Cinema? How does that come into play, or is that a completely separate unrelated system?
Amps kick out heat when they are driven hard, simple as that. That heat has to go somewhere, which means you have to give it somewhere to go. The amp Owner's Manual typically lists the recommended minimum amount of space the amp needs around it. Notice the vent holes/slots on the top of the amp; that is where the heat is and that is where it needs to get out.
Also, curious why you have both? Are you Bi-Amping, if so, what speakers? Which Amp will drive the bass section of the speakers? Or alternately, you might be simply driving 4 separate speaker? These details matter in our responses.
In general, you can do anything you want. However, I would not recommend stacking the amps. If I were forced to stack them, then I would create some type of gap between them. What I have done in the past is take small block of wood, and finish them nicely and place the between pieces of equipment to expand the gap between them.
However, the ideal solution is to stack low powered equipment, and give each piece of high powered equipment its own shelf. You haven't described or provided a photo of your equipment stand, nor a list of your other equipment, so it is next to impossible for us to speculate on an arrangement that might work for you.
But, short of more information, the best advice is to place each power device on its own shelf. Typically an amp need 3 inches on each side and 3 inches above the cooling vents. That can be augmented with fans, but that opens a whole new can of worms.
So, again, you can do what you want, but the best advise is to give the amps some space.
Can they be stacked - yes.
Should they be stacked - probably not.
For what it is worth.
Steve/bluewizard