Things to consider:
1) Execution costs (as you stated, will be higher w/ 2x the trades)
2) Bid/Ask spread costs will be doubled (as you're paying the worse 2x)
3) Contract size: make sure the contracts sizes line up
i.e. AUD future is A$100,000 = $71,800; EUR is €125,000 = $141,875, so if you...