Poodle and Chihuahua problem?

Hi. We have a chihuahua and a poodle with us and they are very sweet. I'm sort of worried because our poodle keeps on on the chihuahua. It's like the only thing our poodle knows is to hump on her(poodle is male;chihuahua is female). They chase each other the whole day.The chase usually starts when the poodle smells the of the chihuahua, chi gets irritated, hides her with her tail and the chasing begins.I get dizzy by just watching them ran all day.They only rest for a couple of minutes and then repeat the chasing again. I am also very worried because the chihuahua may get tired of protecting herself and she's not even a half year old. Shes only 5 months. The poodle, we have no idea because he was only given to us. We don't know what his age is or if he was neutered. The only thing we know about him is that he received a shot/shots for anti-rabies. We are currently waiting for the poodle's paper be given to us by the old owner.We often leave them in the garden to socialize and then tie them when it's time to sleep. Please give us some advice to stop the . we cannot tie one of them because we are afraid one will get jealous. Some advice please. Thank you very very much for sensible answers!!!! :D