How many decks?
I agree with the comments to date, but have also found that the number of decks in play makes a significant difference.
As a general rule, I will rarely increase the number of hands on a double-deck game because of the potential heat, and because many places have a no-mid-shoe-entry rule which also does not permit you to go to multiple hands.
However, on a shoe game, I've never noticed any heat specific to playing multiple spots. This is especially the case if one or more of the following is true:
1) You don't wait until your max bet to switch to two hands.
2) You switch to two hands after losing a few hands to "change the flow of the cards" (although you probably don't want to state this specifically since justifying your actions is one of the things that draws their attention).
3) There are two or more other players at the table, meaning they are less likely to notice how many hands each player is playing at any given time.
4) You sometimes increase the number of hands when the count drops. For example, you bet 1x$100 at a high count, the count drops, and you bet 2x$25.