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