Series connected MOSFETs effect on the channel length
For two NMOS transistors in series with the gates tied together one transistor is in saturation the other one in the linear region.

Based on the simple square law model it can be derived that the equivalent dimensions are
$$
\left(\frac{W}{L}\right)_{eq} = \frac{\left(\frac{W}{L}\right)_{1}\left(\frac{W}{L}\right)_{2}}{\left(\frac{W}{L}\right)_{1}+\left(\frac{W}{L}\right)_{2}}
$$

Applying this equation repeatedly it can be used for more than two transistors.

For two transistors of length L the equivalent length turns out to be 2L if the widths stay the same.