If two species could mix and produce a hybrid it would open up a issue not had between two species. If two species could produce even sterile hybrids would be chance with hybrid vigor that the hybrid is superior to parents. Could produce two different hybrids like lions and tigers can make either Liger or Tigon. So many possibilities and threats that could happen for two species. A species could die out but live on in blood of another species through hybrids.

Two species could work together to produce sterile hybrids to produce better soldiers. Could run into all sorts of issues between two species. Imagine two species both are ruled by kings and they can produce fertile hybrids, exchanging of children could result in leadership being run by hybrids. Would be so many possibilities but would be limited to select cases naturally until technology advanced highly.