Can my co-advisor be from another institution? I started my PhD program and have finished qualifiers already. I had a discussion about topics with my research advisor. However, my advisor admitted he does not know so much about the topic of my PhD. 

Thus I had the following questions:


Can my co-advisor be from another institution?
If so, will this impact my thesis committee?
Yes, it's common in the institutions that I know. Where I graduated, at least one member of the committee even had to be an 'external'.
That doesn't​ mean the same applies to your institution. Read your local examination rules and check with your first supervisor if external (co)supervision is possible in your institution and what the implications are.