You can modify the DataParallel function to send 40% / 60% or 30% / 70% splits across two cards, instead of 50%-50% default split.
However, if you work with multiple model architectures and parameter sizes it will be really hard to balance the workloads across different cards.  Both GPUs may not finish a minibatch at the exact same time causing inefficiencies in training.  I would recommend selling slower card on eBay and replacing it with a matching card, so you can do a 50/50 split on mini-batches.