I don't know why you always talk about formatting.

When you have productive node AIX1 and it's backuped by a standby node AIX2 of which AIX2's storage is a mirror which is at normal times read-only from AIX2 side, then you just don't touch it from AIX2 as long as there is no crash or maintenance needed.

You do not FORMAT. There is no such thing to do that at all. Get rid of that word!
If you keep your cluster maintained, normally AIX2 would know at least all VGs AIX1 knows. If not you can, as vbe already said, just importvg the VGs from the now read-write available storage on AIX2. Also you can maybe just varyonvg them.

Forget that word "FORMAT"!

When storage is available and VGs vary on at AIX2, you can just add a new LV by smit(ty) or mklv + crfs for example.

What we do if a VG changes on one node is, that we get a downtime and varyoffvg it on prod node and just exportvg/importvg it on AIX2 so it also knows/has an up-to-date configuration.
Small hint: There is a switch -L for importvg to learn any changes in configuration compared to what it currently knows; from man importvg:

       -LVolumeGroup
            Takes a volume group and learns about possible changes performed to that volume group. Any new logical volumes created as a
            result of this command emulate the ownership, group identification, and permissions of the /dev special file for the volume
            group listed in the -y flag. The -L flag performs the functional equivalent of the -F and -n flags during execution.
...

You want to have an HACMP functionality without the software (otherwise you would have sai "we use HACMP") - But that doesnt change what Zaxxon and I have explained:
A disk or PV is useless if you dont know its structure (and so you create one!) to be able to "share" the data on it - Since UNIX takes care of the writing to it, only one can access in RW at any time - That is why you have the exclusive mode - to let the OS know it is a shared disk, for this both server have to know the the content of the VG ( the PVs and LVM structure) that is why you DONT CREATE a new one (server 1 did that...) but importvg.
What we are saying is that server2 should already be aware of the VG and you, to mount its content only have to varyonvg, zaxxon will correct me if I didnt get it right...

@vbe: That hits it.
There is also concurrent access to storage in clusters possible to be configured, but it seems that isn't the case here. All of our clusters which are 2 node clusters have due to reservation only access to 1 storage at a time. Writing the mirror is done by SRDF on our EMC system, by AIX mirror on our part of our Clariions and SAN Symphony at the other half of Clariion storage units.
We only have concurrent acces on Heartbeat devices.

Hello Zaxxon, Vbe,

thanks a lot for your answers.
I will test and test for sure !!
Cheers
Ivan