merger (of companies, towns, etc.)

double issue (of a magazine, etc.)

merger

concurrence

combined use

merger

annexation

joint establishment (esp. schools of different levels or different courses of study)

complication

double play

coexistence

writing side by side

ambivalence

uniting

occurring simultaneously

classifying together

reading (two or more books, etc.) at the same time

complication (in illness)

owning together

placing side by side

to have two things (at the same time) (e.g. good and bad points)

to consider together

to use jointly

to be broad-minded enough to associate with various types of people