It's probably difficult for two people to ---> It's probably difficult for others to understand ...
the (many) days the two spent (together);
the days/time they/the two went through together
重ねる is more or less 'gather'. きた is in fact the verb 来た, or 'came'. When you see きた tacked onto the end of another verb, it means 'came to be'. Thus, 重ねてきた means basically the same thing as '...that have been gathered' in this context. Essentially, the sentence is something like, 'It's probably tough for other people to understand the history those two have'.
Also, 二人 is commonly used as a pronoun, for example, 'Hey you two', or 'How have you both been?'. I don't often see 二人 used to count; I usually see 二つ, which literally means 'two (of an object)', for example, the two firefighters, which would probably be 二つの消防士.
* the (many) days the two spent (together); This is the equivalent of Aarowaim's word choice, "history."
I may prefer more standard translations, but depends on the context, variations could be:
* the days/time they/the two went through together-- if they had hard a life/time together
* the days/time they had for each other - if talking about loving relations for example
Definition of くる (きた in this sentence):
Definition 9-イ on the google dictionary link (in Japanese)
二人(が) ... : あの二人； （あなた達）二人； （彼ら）二人 are used for 2 individuals. Always use 人 to count people.
二つの消防士 ---> 二人の消防士（が）, 消防士が二人, 消防士の二人（が）