Tell apart

Dá se říci:

I could not tell apart John and Marry. They look so much alike.

Nesedí mi zde částice apart před předmětem. Spíš bych řekla:

I could not tell John and Marry apart. Děkuji.

You are right. Standard usage:

• 1. I could not tell John and Mary apart.
• 2. I could not tell apart John and Mary. (this sounds unacceptable as a short sentence)

However, the position of „apart“ immediately after „tell“ (as in sentence 2 above) is allowable where the compound object (or object phrase) is long, and putting „apart“ right at the end of the sentence might lead to misunderstanding the sentence, so:

We were in the choir rehearsal room at the National Cathedral in downtown Washington DC last week for her instruction with a prominent professional opera performer and while I was on the sidelines thumbing through some draft translation it occurred to me that at times I could not tell apart the singing voice of my daughter and that of her instructor. (kevinhendzel)

Compare:

• 3. … it occurred to me that at times I could not tell the singing voice of my daughter and that of her instructor apart.

In sentence 3, the speaker or writer might think that the sentence is too hard to understand with „apart“ right at the end.

2.6 Verb transitivity (…) Other transitive verbs require the object to precede the adverb, even when the object is a long noun phrase.

• My husband cannot tell the flowers apart. (not My husband cannot tell apart the flowers.)
• My husband cannot tell the roses and the daffodils which have the same colour apart.

On the other hand some authors say that the verb must be followed by the particle whenever the noun phrase is lengthy and complicated. (page 8, Boháčová, Bakalářská práce)