couldn't be more different
irrelative
Tweedledum and Tweedledee
nondifference
unwedded
disparate
two sides of the same coin
sleeves from one's vest
confusion
spacelessly
legend in one's own lunchtime
nonresemblance
incomparable
ill-matched
have it both ways
hair-splittingly
nonoleaginous
oil and water
nonidentical
unbegun
matchless
unparalleled
much of a muchness
earthbound
undistinguished
duck fart
insignificancy
flea
quisquilian
disconnect
ships that pass in the night
separated
neither
uncompanioned
zero
out of thin air
solitary
matterless
sugar and spice
null
distinctionless
secret
cluelessness
light
betwixt and between
price of eggs
cluelessly
know-nothingness
unequalled
xor