I gave up and used minipages like this:

  newslide(
    content = {
            cat("\\begin{minipage}{0.4\\textwidth}\n")
            print(table2)
            cat("\\end{minipage}")
            cat("\\hspace{0.1\\textwidth}")
            cat("\\begin{minipage}{0.4\\textwidth}\n")
            print(plot1)
            cat('\n\n')
            cat("\\end{minipage}")
    }
  )