How to calculate voltage divider between two transistors?
Looks like you forgot about the MOSFET body diodes.

Since both MOSFETs are cut off, and since both diodes are in series-aiding and are foward biased by V1, current from V1 makes its way through both diodes and (if the drop across them is equal) the voltage at the junction of the diodes, DELTA_V, has to be halfway between V1 and ground, as shown below.


By the way, here's the LTspice circuit list just in case you want to play with the circuit:

Version 4
SHEET 1 880 680
WIRE 432 64 224 64
WIRE 480 64 432 64
WIRE 224 80 224 64
WIRE 176 160 96 160
WIRE 480 240 480 64
WIRE 224 256 224 176
WIRE 432 256 224 256
WIRE 96 304 96 160
WIRE 224 304 224 256
WIRE 96 416 96 384
WIRE 224 416 224 384
WIRE 224 416 96 416
WIRE 480 416 480 336
WIRE 480 416 224 416
WIRE 96 464 96 416
FLAG 96 464 0
FLAG 432 64 DELTA_V
SYMBOL nmos 176 80 R0
WINDOW 0 86 32 Right 2
WINDOW 3 159 58 Right 2
SYMATTR InstName Q1
SYMATTR Value IRFH5053
SYMBOL voltage 224 288 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value .8
SYMBOL nmos 432 336 M180
WINDOW 0 -32 59 Left 2
WINDOW 3 -104 34 Left 2
SYMATTR InstName Q2
SYMATTR Value IRFH5053
SYMBOL voltage 96 288 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 1
TEXT 110 440 Left 2 !.tran 1