It may be related to birth spacing. I tend to look at primitive culture also when I am searching for what is natural, that or even the great apes, as we share so much of thier DNA.
From what I understand, tandem nursing is a modern practice resulting from closer birth intervals, and therefore two children of breastfeeding age. Where as primitive and traditional societies tend to space births further apart, either through lactational ammenorheah, sometimes also sexual abstinence, and likely nurse one child.
This is another MDC on the subject: http://www.mothering.com/community/t/1078480/traditional-cultures-children-spacing-according-to-wapf/20
I read that in 1880, 96% of U.S children were nursed 2-4 years. I am curious what a typical birth spacing was back then and if tandem nursing was common.
One caveat, throughout history, the wealthy or those of royalty, often had many closely spaced pregnancies, as fertility was regained much sooner due to the utilization of wet nurses.
More on birth spacing:
In a study conducted in the West African country of Rwanda, a culture in which there were no contraceptives or taboos against intercourse after birth at the time, there were no differences in the birth intervals of bottle feeding mothers in the city compared to those in the rural areas. On the other hand, among breastfeeding mothers, there were significant differences. Among the city mothers who were already developing patterns of separation from their babies, 75% conceived between 6 and 15 months postpartum. However, in the rural areas, mothers had their babies with them all of the time, and 75% of the rural breastfeeding mothers conceived between 24 and 29 months postpartum. An even more dramatic example of the effects of very frequent suckling is provided by the Kung tribe. (The exclamation point represents a clicking sound.) Anthropologists watched these people with stopwatches and found that the babies and toddlers were nursing an average of two minutes every 15 minutes, and the mothers were conceiving at about 35 months. Such extended periods of breastfeeding infertility are rarely seen in Western culture. First of all, only a few Western mothers nurse that long although their number seems to be increasing. Secondly, there is some speculation that the richer diet of Western women may contribute to an earlier return of fertility.
Regarding the Great Apes, they nurse for many years and birth roughly 5-8 years apart, depending on species. http://tinyurl.com/6lkkxsh I have not read anything about them tandem nursing but they do breastfeed while pregnant but wean later into the pregnancy.