If you've been feeding for 2 months, the food is being consumed, and the worms look happy and plentiful, then go for it. It's not an exact science. Add a tray and put some more food and bedding in it. It may take a week or two for the worms to start moving up into the new tray, so be patient.

I usually wait until I see the most recent food is being consumed before I feed more. If you feed too much it gets very wet very fast. It's easy to overfeed in a Worm Factory.

Below is an excerpt of what the online instructions say. One alternative is to "starve" the bottom tray for 2 weeks and then place fresh food in the upper tray before covering it with new bedding.

The bottom tray will continue to be processed and compress, so you'll need to mound it up into a hill every few weeks so that part of it continues to touch the bottom of the upper tray. Worms don't jump well.

"WHEN TO ADD THE NEXT TRAY:

When the upper tray is converted to a mixture of vermicompost and castings and reaches 2 inches above the inner ribs of the tray then it is time to add the next tray. Scrape an inch of compost from above the ribs and place into the next tray making sure that the top of the bedding in the lower tray comes in complete contact with the bottom of the tray you are adding. Feed and cover with 4 in. of dampened shredded PC paper."

http://www.vermica.com/articles/worm_factory_instructions.htm

Cheers,
Andrew