In dual frame video coding, one short-term reference frame and one longterm reference frame are available for motion compensation. In prior research, it was shown that overall video quality was improved by allocating bits unevenly among frames so as to periodically create a high-quality frame that could serve as the long-term reference frame for some time. We extend this work to a cognitive radio scenario where pulses of extra bandwidth can be rented by the second, but rental agreements can be cancelled on short notice if the legacy user of that spectrum returns. With a scalable video coder, this pulse of extra bandwidth can be used to improve the quality of the current frame being encoded, or of the past long term frame, or can be spent on future frames if the encoder has access to them in advance. We compare these various uses to explore the advantages of allocating some of the available bandwidth to past or future frames.