Translation:On this tour we all walk together!
You can use many adjectives as an adverb, that's what is happening here. But you can't do it the other way around. You can say:
Wir laufen die Tour gemeinsam (adverbial use)
Wir laufen die Tour zusammen (adverb)
die gemeinsame Tour (adjective)
but you can't say
die zusammene Tour (wrong!)
If you use it as an adjective, you need to give it the proper inflection ending: "Die gemeinsame Tour." Just as an adverb it doesn't get an ending. So your sentences are
"Die gemeinsame Tour zwischen den beiden Familien..."
"Die beiden Familien gehen auf einer Tour zusammen."