Pasaport ve bilet ayrıca saklanmalı, yoksa kolayca karışabilir.

Turkish often leaves nouns in the singular when the meaning is already clear from context. Here, pasaport ve bilet naturally means the passport and ticket or passport and ticket in general.

If you said pasaportlar ve biletler, it would sound more like passports and tickets in the plural, which is not the most natural reading here.

Using bare nouns like pasaport and bilet makes the sentence sound like a general instruction or rule.

If you were speaking directly to one person, pasaportunuz ve biletiniz ayrıca saklanmalı would also be very natural and would mean specifically your passport and your ticket. So this is mostly a choice of style and context.

Here, ayrıca means separately.

This word can also mean in addition / besides in other contexts, so learners often notice that it has more than one use. In this sentence, though, the meaning is clearly kept separately.

Ayrı is usually an adjective: separate.

Ayrıca is an adverb: separately or additionally.

In this sentence, the word modifies the verb saklanmalı, so the adverb ayrıca is the correct form.

Saklanmalı can be broken down like this:

sakla- = keep, store, hide
-n- = a passive/reflexive-type suffix
-malı = should / must / ought to

So saklanmalı means something like should be kept or should be stored here.

In other contexts, saklanmak can also mean to hide oneself, but in this sentence the passive-like meaning be kept/stored is the natural one.

Usually it is closer to should, ought to, or need to, but the exact strength depends on context.

In instructions, warnings, or advice, it can sound fairly strong. So here saklanmalı gives the sense of a firm recommendation: this is what should be done.

Yoksa means otherwise or or else here.

It connects the first clause to a possible bad result:

Pasaport ve bilet ayrıca saklanmalı = first instruction
yoksa kolayca karışabilir = otherwise, they may easily get mixed up

So it works a lot like if not or otherwise in English.

Karışabilir comes from:

karış- = get mixed up / become mixed
-abil- = can / may / be able to
-ir = aorist/general present marker

So karışabilir means can get mixed up, may get mixed up, or could get mixed up.

The aorist here gives a general possibility, not just something happening right now.

This is an important difference:

karışmak = to get mixed up, become mixed
karıştırmak = to mix something up, confuse something

Here, the passport and ticket are the things that might end up mixed together, so karışabilir is the right choice.

If you used karıştırabilir, it would suggest that someone or something might actively mix them up.

Turkish normally puts the main verb near the end of the clause.

So:

Pasaport ve bilet ayrıca saklanmalı
and
yoksa kolayca karışabilir

both follow a very normal Turkish word order pattern.

Also, adverbs like ayrıca and kolayca usually come before the verb they modify.

In Turkish, third-person verbs often do not need an explicit plural ending when the subject is already stated.

So Pasaport ve bilet ... karışabilir is completely normal. Turkish does not have to mark the verb the way English often does with they.

A form like karışabilirler is sometimes possible in other contexts, but here it would sound unnecessary. The same idea applies to saklanmalı as well.