The tax law(s) of this country are so complicated the government could say you are denying them payment and you could get a good attorney that demonstrates they owe you a refund.
Just had a friend finish his. He took 5 days to enter in all the transactions. Gracious. I just added mine up and summarize them like I always do. The numbers match and I know someone who works for the IRS. This method wont get you in trouble. Unless your concealing profits of course.
If you could predict the market that well and take a position to get a win in 1 account and a loss in another, the smarter thing to do is to just take the win, and pay tax on it. So that scenario is unrealistic. Now, finding a security with a very wide spread and trading against yourself -- that's a valid scheme.
I think this is where I can plug TDAmeritrade with "gainskeeper" that will do your ticket for you. There is also other software programs that are expensive, but worth it.