How do I give legal guardianship to a step parent without removing the biological father’s rights?

I am divorced from the biological father of my children. However, we maintain an amicable relationship as it pertains to the boys. The boys reside primarily with me full time and see their father every other weekend. I have no desire or interest to affect his current parental rights, but both the biological father and myself would like for the boys step-father to be able to have rights to do things such as take the boys to doctors’ appointments and school meetings and sporting events and such. What are our options for that? Is that possible?