this is not possible. You have to copy the files to the folders if they already exist. But you can choose the secondary drive as a target during import. An even better - you can save your import settings as a preset. Define two presets e.g. "new images to 1st drvie", "old images to 2nd drive" an choose the correct one during import.
If you have not already imported them to the "other" hard drive, importing them to the desired drive would be your best bet.
If the photos are already on two drives, you can merge them but not entirely within LR.
First you would need to use Explorer/Finder to merge the to groups of files into one directory tree on the one drive.
Then you would go into LR, Library module, Folders panel, and "relocate" the moved directories.
Test this with a small set of files to make sure it works the way you want it to.
I just discovered something. In the left panel, in the folders section is the original drive letter, grayed out. All of the photos are there, no image, but the metadata, and keywords with the "!" in the upper right corner. When I right click on Find Missing Folder, I go to the current drive letter where everything is. Remember, its the same drive, only the letter has been changed. The folders are organized by year.
When i click on Select Folder, same name/year as it was before the drive letter was changed, I get the following message: The selected folder or one of its sub folders is already in LR. Do you want to combine these folders?
The sub folders are months and days.
If I select Merge, will the keywords stay with all the images when I merge the folders?