This Tasker works with two profiles. Together the two profiles will set the phone to vibrate when you receive multiple text messages within 2 minutes, and will return the volume to normal after 2 minutes of not having received a text message.
So, if you get a text message, then get another text message within 2 minutes, the phone is set to vibrate and the 2nd message will only vibrate, not play the notify sound. After the phone is set to vibrate, each new text message resets the countdown time period, so you continue to have the phone on vibrate until you have a 2 minute time period of not receiving a text message. Also, the phone tasks won't run if your phone was set to silence/vibrate before you receive the first text message (so there's no chance that this will unsilence your phone when you expect it to remain silenced).
Note that the phone isn't set to vibrate until it actually receives the 2nd text message, and yet (for me on my droid razr maxx hd at least) the 2nd message is still silenced.