If the two people have been tested and know they do not have any sexually transmitted infections, then an sti cannot be passed between them. However moving a penis or a sex toy from anus to vagina is not advised unless condoms are used and changed inbetween penetration. This practice is not recommended because bacteria normally occuring in the  anus can be introduced into the vagina and cause infection for the woman. These bacteria can also cause an infection in urethral opening and the tip of the penis.