box and cox
coalternate
intervolve
interden
one-on-one
interlace
interchange
double-handedly
pendant
doublet
two-handed
battle of the sexes
cross-handed
alternize
discord
disaccord
have a foot in both camps
co-
pair off
disagree
better
divide
doubly
six and two threes
vagrate
pass
twosome
apart
duet
couple
duo
set up
multialternative
dispair
in between
shuttle
chemistry
on the horns of a dilemma
to and fro
as the case may be
whatever the case may be
biune
codual
surrogatum
interoptode
opposite
differ
bothridial
tweely
'tween
drive a wedge between
interdistinguish
diarchy
bind
between
split
unwed
ambidirectional
duad
third-wheel
go-together
dichotomize
coexperience
interpersonal chemistry
betweenity
wherebetween
alchemy
interpermeate
incrustate
varify
alternative
prelation
mixed
intermental
appose
obverse
subrogate
set off
two-time
odd couple
selected
discern
understanding
twinstick
Tweedledum and Tweedledee
stand in for
reverse
Tweedledee and Tweedledum
wear two hats
tandemwise
double
duopolistically
dualistically
coexchangeable