Possible Clues:

• On the same page
• A Liberal proved untrustworthy in coalition
• Joined for mutual benefit
• Joint American and Labour leaders misrepresented
• In a league, say
• Joined by, eg, treaty
• Everyone gets to die - that's something like
• Matched, beheaded yet joined in some way
• Word in many union names
• Like Cuba and Venezuela, e.g.