trading in a farther out contract is an entirely different story. Regulations concern themselves with the long and short position in one and the same contract and that means for futures same symbol, same contract expiry date. Second brokerage to my knowledge is entirely legal and not discouraged. But brokers that are regulated in most developed jurisdictions are prohibited to maintain concurrent longs and shorts without offsetting one side first and in effect closing one position. No regulated broker in such jurisdictions can show a long and short position at the same time in the exact same asset in the same account. Of course can I easily buy asset1 for account1 in the same broker and sell asset1 in account2 of the same broker. But I cannot do so in the same account without affecting the original position in the same asset.
I'm pretty sure there's nothing illegal about it. I've contacted brokers about it and they've told me either use a farther out contract for the second part, or use a second brokerage. This can be done for a lot of reasons. What if your platform is down at one brokerage and you want to get out but you can't? You could simply take the opposite side at your other brokerage to protect your capital. I think it's a smart thing to do.