This is more of an oddity than a problem for me. I am using 12.0.4 LTS and have hfsprogs installed. (I don't have any reputation so I can't create a tag for hfsprogs, and that it doesn't already exist makes me think very few people will care abou this.) I have been able to mount one of my external Mac hard disks without any problem. I am slowly moving everything to my Ubuntu installation from my old Mac installations which are backed up on the external hard disks - formatted as HFS+. I have been keeping one plugged in most of the time. I then plugged a second one in and it would not show up until I unmounted the first one. I don't know if this is a bug or if there is a problem I should know about. I am guessing relatively few people need two HFS+ formatted drives mounted simultaneously.
I am using an Ausu N76VZ that I try to keep current: $ uname -a Linux N76VZ 3.2.0-38-generic #60-Ubuntu SMP Wed Feb 13 13:22:43 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
I saw at least one other question about mounting two external hard drives at the same time - I have no idea how to tell if this is the same problem, meaning mounting two external drives at the sime time is a problem with Ubuntu, or with hfsprogs, meaning it is an HFS+ specific problem. You might want to look at Can't mount two external hard drives at the same time