Installed combustion today. After looking briefly thru the manuals and the app menus I couldn't find any tools to deinterlace / field merge video. My error or discreets 
?
I'm also looking for a tool like that....
Not our error, I think! 
Just set the Field Separation appropriately for your source footage (in the Footage Controls tab with the footage selected). De-interlacing is then handled automatically as needed.
If you need to then create a de-interlaced clip set your composite settings to have no fields.
If I have a sequence rendered into seperated fields... How can I merge them using Combustion*? For now I have to use Shake to merge them, then ouput to an interlace sequence and load this sequence into Combustion*.... What a time waste!
Import the field rendered sequence as double the frame rate of your final composite. (ie footage setting for the framerate is 60fps.) And make sure you specify fields in your composite and render settings.