1. Take a shot of a clock (such as my computer's clock) with each camera. You can do this before or after taking the photos. You don't even need to do it on the same day, although ideally you would do this before importing the photos. Note the exact time shown on the clock in the clock photo (not the file's timestamp).
2. I import the photos into Lightroom (including the clock photo).
3. Select the photos from that particular camera only. There are several ways to do this:
a. If you imported the photos together with the clock photo, you just have to select everything that you imported.
b. If you imported the photos separately from the clock photo, switch to library view and change the sort method to added order instead of the default capture time. Then you can easily select the photos that belong to a particular camera. Add the clock photo to the selection using ctrl+click.
c. You can also use a metadata search to look for the photos from a particular camera.
4. Switch to the Library module. While still keeping the particular camera's photos selected (including the clock photo), highlight the clock photo by clicking on it.
5. Click on the Metadata menu option and select "Edit Capture Time...". Select the first option, "Adjust to a specified date and time". Specify the time shown on the clock. Click on "Change All". (Note: if it says Change instead of Change All, you most likely deselected the other photos. Go back to step 3.)
That's all there is to it.