I’m new to Sikhism and my Muslim friend asked how it was possible for there to be two living Gurus at the same time—if their lives overlapped, were there two Gurus at one time? If you could help clarify this, I’d appreciate it, as I wasn't sure how to answer and don't understand Sikhism that well yet.

First of all, Shabad has been the Guru, from the very beginning, since the First Guru Person, Guru Nanak Sahib, had made it absolutely clear, during his discussion with Yogis, that his Guru was Shabad when he said Shabad Guru Surat Dhun Chela’ (ਸਬਦ ਗੁਰੂ ਸੁਰਤਿ ਧੁਨਿ ਚੇਲਾ). Therefore, all along, Shabad has been the Guru of Sikhs.

Now about the Ten Guru Persons, they were considered Gurus, because, they were spiritually embodiment of Akalpurakh. The body was not the Guru.

At any point of time, in Sikh history, two Gurus were never present, because though bodily they were present, at one point of time, but only one was considered as the Guru and the other was not, till he was ordained as Guru.