I skimmed and read carefully some sections of JD's exposition** of uncertainty (sig. figs, etc.) and didn't find an answer -- suspecting it may be there please point. OTOH, if not, people help.
First, here's my idea [assuming the pols are scientific, i.e. at the same time, random, unbiassed, those poled are likely voters, etc.] The error is due to being a small sample of a large pop. So several polls, when combined, are a larger sample of the same pop. Therefore, the uncertainty of the combined polls is less than the uncertainty of any of the individual ones, no? And again, how does one calculate it?
** Measurements and Uncertainties versus Significant Digits or Significant Figures
bc wishes to reply to a newsletter that claims one can't have a combined error less than the lowest individual one.