How can I schedule a backup at 11:00 am to d: and a backup at 11:00 pm to g:?

I'm sorry -- you're going to have to give a lot more details about what you are trying to accomplish.   Like what's the source of the backups, what's the target and where is it located? 

Simplistically, you'd have to create 2 backup Jobs, one for the D: backup and one for the G: backup.  After you created the jobs, you'd have to create a schedule for one to start at 11 AM and the other to start at 11 PM.

^ Correct.  Even if you're backing up the same source data at both times, if you want to make sure that one time always uses Destination A and the other always uses Destination B, then you would have to use separate definition files.  You can just duplicate the one you already have, then switch the destination in the duplicated definition file and make sure the schedules for each are correct.

HOWEVER, if you can safely assume that only one of those drive letters will be available at any given time, you could instead use Reflect's "Alternative locations" feature.  You'll see a link for that directly below the field where you specify a destination.  In there you can specify fallback destinations.  If you do that, then you can schedule both job run times in a single definition file, because each time the job runs, Reflect will back up to whichever destination is available at that time.  If both are available, I believe it backs up to whichever destination is higher in the list.

If the source and destination are the same at 11 a.m. vs. 11 p.m., but the external drive letter has changed because other storage devices occupy D:, E:, and F: when at home, then Volume GUID based discovery would also provide a solution.  

Dan's response is the correct one -- the question cannot be answered without additional infomraiton.