Derive both MAC and AES keys from same PBKDF2? When doing password-based encryption, is it OK to create one instance of PBKDF2 from the password, and then use it to create both the AES key and the MAC key? (Or should a new instance of PBKDF2 be created using a different salt or iteration count, for each?) 

This answer   seems to say this isn't a good idea. While this one   seems to say it is. Am I misunderstanding one of them? (Or both.)

(And by the way, the first is marked as a duplicate of a question which deals with an initially strong key, while it itself deals with a simple passphrase.)
From a security point of view, deriving lots of key material using PBKDF2 is ok.

From a practical point of view, deriving lots of key material using PBKDF2 is inefficient (in the sense that to generate $n$ blocks and increase the adversary's work by $t$, you do work $nt$, instead of $n+t$).

A more practical solution uses PBKDF2 to generate a short string and a suitable generator (probably HMAC-based) to produce lots of key material. Typically:
$$k_0 \leftarrow PBKDF2(pw, \dots)$$
$$\text{key material} \leftarrow HMAC(k_0, 1) || HMAC(k_0, 2) || HMAC(k_0, 3) || ...$$