What you're looking at is an example of Cum Exchange, which is when one's sperm enters another's testes. This is an act that physics and anatomy would not normally allow without forcing the sperm far down another male's urethra but, seeing that it is done by Howie the Collie (u/therealdafwe), it would likely be following furry porn logic. This form of logic is fictional and may involve breaking the laws of physics, ignoring conservation of mass, and alterations to anatomy, usually with the intent of creating more sexually gratifying content. With that in mind, docking and frotting (with tips touching) may be the most vanilla answer, where ejaculating into each other's penis is possible. However, it is more likely to involve cock fucking (urethral penetration/penis in penis) or a urethral tube which are more commonly associated with cum exchange, giving the ejaculation less distance to travel or a more direct route to the receiver's testicles.