Say opener introduces a second suit, showing a five-four shape. What now, for these dustbin responders, after: 

1♠ - 1NT - 2♣ - ??

• i) Pass. 2♣ may not be especially pretty, but bidding 2♥ is likely to be worse. Given that partner holds at most four red cards, you'd not be surprised if he held a singleton heart. As we'll see in (iii), if you introduce a new suit after bidding the dustbin lNT, you should have six cards.
• (ii) 2♠. There are two very good reasons why you should bid 2♠, even though you have more clubs (a). A 5-2 fit normally plays more easily than a 4-3 fit. (b). Bidding 2♠ (as opposed to passing 2•) gives partner another bid, should he hold say 17 points (perfectly possible). Bidding 2•, even though you prefer clubs, is termed 'false preference'.
• (iii) 2♦. This is not a suggestion. It is virtually a command for partner to pass, based on a six-card diamond suit. With a diamond fewer, you'd go with one of partner's black suits.