passerby wrote:You could mount /var/cache (or, more specifically, /var/cache/apt) on a separate partition.
When you download updates, they stay in /var/cache/apt/archives until you remove them.
If you have both installations mounting the same /var/cache partition, you'll only need to download updates once, then apply them separately on each.
I actually have several different computers, not several distributions on the same PC.
But so if I downloaded the updates on 1 computer I could just copy the contents of /var/cache/apt/archives onto my other computer and install updates without downloading, right?
Where do the install files for programs download to when I install them?