The first consideration is, in how far would a debian sid x64
installation differ from the analog x32 install at all? Can i track it
down to some few directories, like /bin, /sbin, /lib ... what else?
Even if you can, you still need to keep both installations up to date.
So mixing e.g. /usr/share means that it would be out of sync during an
upgrade. This may confuse the configuration scripts, and it is probably
not worth the hassle. You can only save a few GB anyway.
You may want to have a separate /boot partition with grub on it, but I
am not sure how you have to set that up. Maybe mount /boot/grub?
/home .... can there still be differences e.g. in version between x64
There should not be any difference (of course bugs do happen), and you certainly want to share your data, so no choice here.
So put /home on a separate partition (which is recommended anyway), use
two separate partitions for32bit and 64bit. Anything you want to share
can be moved and symlinked to the home partition.
/var/cache/apt/archives ... i can't see any problem here, can you?
Good idea. It will save you double downloads of arch all
packages, although that is not quite as much as I would have hoped.
/usr/share .... a relatively huge peice - but will apt refuse to manage
(mainly, to deinstall) stuff that was updated by the 'other debian' ?
(Would at least /usr/share/doc be safe ?)
Exactly. I would not share it, unless you are really short of disk space. And then I would go for 32bit only anyway.
/usr/scr .... i will have several source trees from kernel.org
debian, and i've no problem with backups of different .configs, so
basically this should work...?
I think you want to keep the kernel sources separately, because make creates quite a few architecture dependent things there.
I think i can share swap (with an additional hibernation swap for each
Yes, unless you want to use suspend. But you can always use swap files, this is not a big problem any more.
I also think i will use tmpfs.
Good idea, just add more swap :-)