Can i run mining on one wallet at more than 2PCs?
Please clarify your question? Do you mean you are pointing 2PC's at the same address? Like -miningaddress="address" bc this is just fine to do.. Do you mean that you want to pool both PC's resources to the same wallet?
i mean that i will copy this wallet.dat to PC2 and run PC1, PC2 at the same time

I'm doing it this way and it works fine.
for me to
Hashes are combined?
I'm also using same wallet on 2pcs and, so far, no problems.
I believe the hashes are not combined, its just like 2 solo cpu mining with mining rewards being sent to the same wallet.
CH
Yup that is all that happens. No need to manage multiple wallets.
Ok you are all more then welcome to do as you wish.. but it is a long known problem to do this..
If you would rather it come from a different source
https://en.bitcoin.it/wiki/WalletIt is intended that a wallet be used on only one installation of Bitcoin at a time. Attempting to clone a wallet for use on multiple computers will result in "weird behavior"
If you would like have more then 1 PC mined to the same address please use miningaddress="address"
Edit: While thinking about this a little more.. I truly think all your doing is using from the same initial Keypool.. So once those 100 Key pairs runout each wallet should generate a new set of 100 which will not be the same on each PC.. Once again I will give you a link to read
https://en.bitcoin.it/wiki/Key_pool