I understand a trader legally can go long in ES and short in MES at the same time even with the same broker. Or two different brokers. Why? because they are two different listed instruments.
i recall we had a conclusive thread on this subject, the answer is yes for both, you can go long and short at the same instrument on different accounts. ask @Overnight
Mike, thanks, and the above is what I thought. And as I believe, every trader is given a number similar to a Social Security number, so whichever Exchange you do business on you're tagged with that number. So if you're long with IB and short with IronBeam, the CME will know. Almost certain they won't worry if it's a sort of one off, but if you do it repeatedly almost certain problems. Remember thr Algorithms, we can't escape...
it could be used as a sneaky way to transfer money from a taxable account to a retirement account eg. have an SPY position in your IRA and a SH position in your taxable account. Can also be used for international money transfers. There are probably laws that frown upon this ....