I'd like to extend Aleks G's answer. Another quite close example is они (как) одно целое or они как две половинки (одного целого). The first one esp. good for things which are not alive. The following works for things as well:

• Идеально подогнаны
• как тут и было (when a new part was added)
• как родное (e.g. after something was repaired)
• как влитой

For both things and people: два сапога - пара

For people: водой не разольешь or закадычные друзья

Joke/informal about two things or even people which are (practically) the same: из одной бочки наливали. This originate from a joke:

A client is asking barman who is swindle but not very smart:

 

-Why 3-year-old wine has better taste than 5-year-old?

 

-Have no idea, was poured from the same barrel!

I'd like to extend Aleks G's answer. Another quite close example is они (как) одно целое or они как две половинки (одного целого). The first one esp. good for things which are not alive. The following works for things as well:

• Идеально подогнаны
• как тут и было (when a new part was added)
• как родное (e.g. after something was repaired)
• как влитой

For both things and people: два сапога - пара

For people: водой не разольешь or закадычные друзья

Joke/informal about two things or even people which are (practically) the same: из одной бочки наливали. This originate from a joke:

A client is asking barman who is swindle but not very smart:

-Why 3-year-old wine has better taste than 5-year-old?

-Have no idea, was poured from the same barrel!