RFC 3264 chapter 4 "Protocol Operation"
The agent receiving the offer MAY generate an answer, or it MAY reject the offer.
So I read from this: either answering or rejecting. Both in one act is not possible.
Let's go to chapter 6 "Generating the Answer"
An offered stream MAY be rejected in the answer, for any reason.
So what is difference between rejecting offered stream and rejecting the offer? Please note that rejecting offered stream is apparently made in answer and is no case rejecting the offer, according to former RFC "either answering or rejecting" applies.
If an agent receives an SDP offer in a Sip request message it can reject the entire offer by returning a Sip response message with a failure final response (4xx, 5xx, 6xx).
If an agent wants to accept (part of) an SDP offer it will construct a SDP answer. The answer is constructed using the offer as a basis: it has the same number of media lines (stream descriptions), in the same order.
For each stream in the offer the answering agent can specify whether it accepts the stream or not. If the agent rejects a stream it can specify this by setting the media line's port value to zero. If the agent accepts the stream it uses a valid port number.