Dear Sadhus,
If they will be idle for 15-20 days, then you may ask them how they would like to fill this time gap productively? Let them brainstorm and come up with their own solution. Hope they are all software engineers, therefore graduates in Engineering.
Secondly, large number of soft skills can be learnt on one's own. There is no need to "teach" as such. As such they are engineers and their IQ level supposed to be high. Self-learning is always best learning. You may identify top 5 behaviour competencies and tell them to master these in 15 days. Let them refer any book or web resources on their own. Knowledge of following competencies can be acquired on one's own:
a) Listening Skills
b) Questioning skills
c) Decision making skills
d) Conflict handling skills
e) How to give feedback and how to receive feedback
f) Interpersonal skills (with emphasis on how to criticise and how to accept criticism)
On 15th day or so, you may conduct their test to know to what extent they have mastered these competencies. Even if they have theoretical understanding of any competency, it is more than sufficient. They may put on use this theoretical knowledge when their projects start.
Alternatively, you may tell them to go through Stephen Covey's book on 7 Habits. Conduct the test for each habit. Trust me, if they master these habits, they will bring big cultural change in the organisation as change in their behaviour will be quite perceptible. But do not just tell them to "read". You must conduct test. Else nobody will show interest in mastering anything.
Secondly, through this method, you will be able to foster learning culture in the organisation. This is far more important. However, to do all this you should be competent enough to handle this. Lest it may do more damage than good.
If you adopt this route, you will find that you have saved lot of resources in training them.
All the best!
Dinesh V Divekar