Find all values of $b$ for which the equations $1988x^2 + bx + 8891 = 0$ and $8891x^2 + bx + 1988 = 0$ have a common root.

Enter all the possible values of $b,$ separated by commas.
Let $r$ be a common root, so
\begin{align*}
1988r^2 + br + 8891 &= 0, \\
8891r^2 + br + 1988 &= 0.
\end{align*}Subtracting these equations, we get $6903r^2 - 6903 = 6903 (r^2 - 1) = 0,$ so $r = \pm 1.$

If $r = 1,$ then $1988 + b + 8891 = 0,$ so $b = \boxed{-10879}.$  If $r = -1,$ then $1988 - b + 8891 = 0,$ so $b = \boxed{10879}.$