Hello

I'm painter too. Exchanging art for cryptocurrencies is great idea.
More i look into this topic more i see more possibilities for building new things based on Bitcoin.

One thing is trading real goods for Btc - like agent between customer (paying in Btc) and printer (accepting fiat money), another thing is building new services just because cryptocurrency made them possible. This new possibilities made me looking for more knowledge on Bitcoin and safer trade.

Thanks, now i understand it more less. So using two wallets with the same private key would be something similar to attacking myself.
Question of multiuser wallet still seem to be interesting.
Unfortunately i've found only one attempt in bitcoinj Google groups. No real conclusion.

Imagine situation where A and B use the same private key/wallet.dat - can it be compared to using one bank account by two persons?
For example A & B have one bank account, each of them hold own credit card and can use money in this account until they spend all.

Is this possible?
Would it be reasonable way of reaching such goal with Bitcoins? (imagine company or family account or similar situations)

One thing that bothers me is double spending. When it will occur:
1) When A and B try to spend more bitcoin than they have in wallet
2) When A and B try to spend bitcoins at the same moment, even if they don't spend all money in their wallet

Which one is true?