These images (taken from the FE13 section of this site) seem to clearly depict the two as fighting each other. Others seem to depict them as allies - I believe the video showed them fighting back to back against some common enemies, helping each other rather than fighting. Others, such as this "swords crossed" one and the box art, are more vague, and seem to depict them as somewhere between the two, which I think will turn out to be closest to the overall truth.