An away rotation is viewed as your sign of commitment to a given field. Doing two away rotations in two different fields makes it obvious that you're not committed to either field yet. While this is the reality for many students and a reasonable situation to be in, it's a terrible idea to broadcast your last-minute uncertainty. There is a real and needless risk that it will be taken poorly and put you bellow similar applicants who don't broadcast doubt. Do the rotations at your home institute and then do the away in the field you ultimately choose.
You don't show up at a job interview before you put your pants on at home...
Edit: You're one of dozens of relatively homogeneous applicants to any given program. Nittpicky bulls--t and random whims are going to count because that's all there really is.