I was told to save 2nd molar tht need root canal &pull bicuspid that needs one too. They said my teeth would lean if I pulled either one out? Whys tht

Shifting teeth. Teeth can shift due to forces you tongue places on them during swallowing and lack of an antagonist.
My recommendation. If your tooth is restorable, have the root canal and save your natural tooth. The fact is when one tooth is lost, the teeth next to the vacated spot begin to shift. Nothing is as good as your natural tooth!
Teeth are dynamic. People understand that teeth can move in a controlled fashion with braces or other orthodontic means but they can also hyper erupt, twist and lean into adjacent spaces on their own. Not everyone's teeth do but it is possible and that is a risk when teeth are removed. Multiple dental opinions may give you more insight into your particular situation. .