The message
from Jonny contains these words:

On 11 Apr 2004 12:52:18 -0700, (Heather)
wrote:
Cut potato
(Wilja apparently) into two bits and put cut sides down, shoots facing
up.(snip)

Hi Heather, Sounds fun. Three things spring to mind about cutting the
spud in two. Firstly, does that work?

Yes, everyone used to do that a couple of generations ago, to get the
maxumum crop.
Dipping the cut side in (cool) wood ash prevents any rot.

Secondly, why do it?

Because two separate sets of foliage will develop two separate root systems.

If the
answer is to get two plants and therefore a heavier crop, are you not
cheating:-)) quote from above "heaviest crop of potatos
grown from a single tuber" emphasis being on the word single.

I'd take the rule to mean "starting with one tuber", and divide it.
After all, when doing that fundraiser where kids have to invest £1 to
make as much money as possible, it would be perfectly okay to invest 60
p and 40p in two separate ideas.

With two potato plants in one pot competing for the same compost, I'd
consider liquid feeding with comfrey or nettle tea.

Janet