Now mind you this was back in the 80's when I learned this stuff (a bit rusty) but a great prof trained at Univ of wisconsin--madison taught reproductive physiology. Fascinating and practical stuff, so at one time I could tell you which mammals had Os penis's (bone) the walrus and dog come to mind, the boar pig has a corkscrew shaped, and a sow pig a bicornuate uterus (two separate uteri)the bear sow can hold onto the zygote and stop the gestation process for a length of time, i think even till next birthing season depending on body fat/nutritional status. I think the possum can do the same, the cat can be fertilized by multiple toms and carry them to birth. But you are right I can't double book two dogs.
