Perhaps ATR kept the 2 engine climb performance out of the book so that people only had one option - use the 1 ENG climb performance. And if you cannot make it on 1 engine - you don't need to know if you can make it on two.
This is of course assuming that we are talking about obstacle clearance performance. If we are talking en-route climb, there is no excuse for ATR not to have made that available.
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