Replace 2 drives at once?

I just ordered two WD 2TB drives to replace two 500GB drives in my Drobo FS, and I’m wondering how to proceed. Can I take both 500GB drives out at once (I should note that I’ve got dual-disk redundancy turned ON), or would it be safer to swap one at a time (i.e., take out a 500GB drive, put in a 2TB drive, wait for relayout, then repeat with other 500GB drive)?
Also, when I take out a drive, do I have to wait for the yellow and green flashing to stop before inserting a new one, or can I insert the new drive immediately?

Replace one drive at a time, and always wait for flashing green/yellow to stop before removing another drive.

You could remove two drives and not lose your data, but relayout is a drive-intensive process, so that would be akin to choosing to drive with your airbag disabled and without your seatbelt fastened. Don’t risk it.

Ok, thanks. Another quick question: I assume that the lights will begin flashing as soon as I remove a drive. Does this mean I have to wait for them to stop before inserting a new drive, or can I insert the new drive immediately?

Flashing green/amber means do not remove a drive from that bay. The empty bay should be blank or solid red (depending on how full you are). So you can insert the new drive immediately.

You can insert the new drive immediately, though I like to wait until the lights stop because there’s a tiny chance that you might dislodge another drive in the insertion of the new drive.

Docchris(?)'s advice of inserting the drives with the Drobo powered off is a good one.

Yes you can do that too.

Shut down drobo, remove drive, insert new drive then power back on.