It doesn't contradict the letter of the game rule, so I think it's hard to ban it without talking about it first (at the start of a game, for example).
On the other hand, I think it goes against the spirit of the rule. Why is that? Because the main aspect of Just One is to find a clue that's obvious enough to help the player, but not too obvious to avoid duplication . By playing a clue composed of two words, you lose this aspect, because there's almost no chance that a pair of words unrelated to the word to be found will end up doubled. It's a different game. It's almost like playing Dixit without the main rule ("if everyone bets on the correct card, the storyteller loses"). Well, not that far, but...
What I find a pity is that people who play with connected clues (the majority of experienced players on BGA in my experience) do so throughout the game, and not just once in a while. What should have been a "think out of the box" moove to spice up the game becomes a routine that undermines the rule, and that's a bit of a shame.
