If we connect two MOS transistors in parallel with W/L , the resulting is equivalent to having 2(W/L)....
Whether the following will hold good in saturation region of operation.... "connecting two W/L MOS transistors in series will be equivalent to having W/2L...."
Yes, when you add two transistor in series while their gate are connected together, then the bottom transistor is always in triode and the upper one depending on it's drain voltage can be in saturation or triode. I recommemd you to prove your second statement, for two cases, that I have mentioned.
If both transistor be in deep triode, you can prove it by using equivalent on resistance of them.