At 12 yrs old, I guess they are pretty comfortable with the pecking order they've worked out for themselves. My concern is that this seems to be a new behavior, and it is aggressive. Since this can climb into other ugly behaviors, I would want it to stop. If you want to be in charge, you should be able to leash whomever you want whenever you want. It would really bother me to have my dog dictating that the other dog has to stay on the deck until SHE gives permission for her to join you.

I would address the growling, make Ms. Bossy lie down & stay down until she settles down, then leash Timid and then Bossy. Don't coo over Timid, just be business-like. If during the time you are leashing Timid the other starts up again, I would put her back down, and wait until it stops. It may take some time the first few times, but she'll get the idea quickly that she needs to knock it off.

Good luck!