Can you Put New Decking Over Old Decking?

No; this is not a recommended practice in deck building. In many replacement jobs, joists end up being repaired, replaced,  or capped. This is due to the common discovery of decay underneath the decking area, usually where the fastener penetrated the joist
If new wood is installed on top of other decking, there will be little drying potential beneath the new decking and more rapid deterioration of both decks can be expected.
We always recommend speaking to a decking specialist for advice in this type of situation!