If I have torn my acl, does this mean I probably have a torn PCL too?

Not necessarily. It is more likely to sustain a rupture of the acl than to sustain ruptures to both ligaments.
No. Although the PCL may be torn with the acl, it doesn't have to be. If it is, its a sign of a high energy injury and your knee will be a lot more mechanically unstable than if you have an isolated ACL or isolated PCL tear. An MRI is the best way to tell if one of both of the ligaments is torn.