Them VS Those

It is correct to say "Can I have two of them?"

It is also correct to say: "Can I have two of those items?" and "Can I have two of these/those?"

Actually pronoun 'they' is also plural of 'it'.

While 'This/That/These/Those' are demonstrative pronouns.

Either them or those can be correct.  Them is the objective form of they. They is the plural form of he, she, or it.  Those is the plural form of that.  This and these are pronouns that point to something close by, something that is here.  That and those are pronouns that point to something further away, something that is over there. 

The customers could point to the items on the shelf and say, “They are what that I want.”  They is the subject.  Making it the object would be, “I want them.”  If you had a bushel of apples, a customer might want two of them. 

Since this, that, these, and those figuratively point at something in a place, it sounds better to use them when a person is literally pointing as well. 

When you add the word items, those and these become adjectives.  Them can never be used as an adjective.  “Them items” is incorrect.