Which two gospel books were written by apostles?

The books of Luke and Acts, both written by Luke, were thought by some to have been written as one work in two volumes. Luke himself makes clear that they are both written to one Theophilus and the subject matter follows on from one into the other. In Acts 1 Luke refers to 'the former treatise' (the Gospel of Luke) which suggests that they were two separate works although they could have been in two volumes or certainly in two scrolls.