What happend if I share my private key with my partner? Can we spend bitcoins together without collisons from the same wallet? Me and my friend are using BlockChain.info - what will happend when we import same private key? Can this destroy this wallet?
Depending on the client you use, this could be a very bad idea, the reason being that some clients send change to new addresses while others send the change back to the sending address.
If you use an online wallet such as blockchain.info you should be fine as it sends change back to itself, and in this case you'll both be online so your clients will always have an up to date list of the transactions for that address.
The chance of having a collision (aka: Double spend) is very low as this requires you to both spend the same transactions within seconds of each other and if you are both using the same online wallet it would probably need to be done near simultaneously.
Remember that the private key fully controls ownership of the coins (hence being private) so you need to fully trust anyone with knowledge of it.