try introducing a long rope toy into the mix.. get the more submissive dog to play tug of war with you then hand over the end your on to the less submissive dog and let them play together.. or maybe toss two balls at the same time .. some how get them in a play mood and playing weather together or by themselves but at least next to each other and at the same time.. this will help a lot for them to get aquatinted and more friendly with each-other

keep me posted on how that works .. I have more suggestions if needed