Is it possible to make co-operative chambers?
In the new Portal 2 Perpetual Testing Initiative is it possible to create chambers that are playable in the co-op mode?
If not at present, has Valve made any announcements to introduce this?
No, it is not possible create co-op chambers with the editor released with Perpetual Testing Initiative DLC.
Valve is working on this (I forgot now where I read it) but no official release date has been announced.
In the meanwhile you can use PTI editor to create the map and then modify it with Valve Hammer editor to make it ready for co-op.