I shared a cat with my ex that we had together for 3 years. There was no way I could handle the emotional damage of seeing my ex so much if we shared her. Plus the kitty was really attached more to him than me so out of love for her, I didn’t fight trying to share or keep her. It was very sad but if you think of what’s best for your pup, it’s going to be to have one owner unfortunately:( I also had a close friend try to share their dogs with an ex and that failed. It gets too messy and confusing seeing an ex so frequently and more than likely you’ll end up fighting over the situation. Wishing you the best, I’m sure this is heartbreaking