If you're 75 gallons and up you're ok, if not your tanks likely too small, and they will start to scrap. Sometimes you're lucky and they'll ignore each other due to colour differences, but likely not,especially as lemonpeels are pretty scrappy apparently. Also depends on how much live rock, cover, aquascaping blah, blah, blah - remember 'out of sight, out of mind...' Significant size differences help too