Good upstreams will cooperate with you. Simply not true. One thing that disturbs me is the thought of round-robin routing over two links that end up in different places. This is a bad thing. Consider that A and B have different latencies.
Packets arrive at their destination out of order.
This will screw up UDP, making it think that there are dropped packets. If you're not running SACK, or have too small of a buffer window, then you'll end up with retransmits. Just make certain that the two share routes properly.