A key-derivation function that is as strong as the stronger of PBKDF2 and scrypt
scrypt uses PBKDF2 internally, so it's absolutely crucial to prevent nasty interactions. My suggestion would be a simpler scheme (using simplified syntax):

$k = \mathrm{scrypt}(\text{key}, \text{salt} \mathbin\| 0x0) \oplus \mathrm{PBKDF2}(\text{key}, \text{salt} \mathbin\| 0x1)$

This does exactly what you want - that is, the output key has exactly the strength of the stronger of the two, without nasty interactions. Your construction may or may not have the same property, but the close relationship of scrypt and PBKDF2 makes me rather nervous.