Not always true...
I have had more than one casino pit refuse allowing me to spread to two hands (SD/DD, mostly DD). I did it at one table, dealer said nothing, the other player at the table (one of those "flow of the cards" morons) said "hey, there is no mid-shoe entry allowed here, see the sign? You are screwing up the flow of the cards..." The dealer and pit boss agreed. I pulled back the second bet and played on...
I thought it was stupid, since I could have just bet more on the single hand, but I wasn't going to make a big scene since I didn't want to explain why I was doing it (to eat more cards in a + count since there was a second player at the table). :)
Hasn't happened very often, but it has happened. I don't even remember where this was, but since I mainly play in MS, and occasionally in Vegas, Laughlin, or Reno, it had to have been in one of those locations most likely...