I think you will need 2 unless you want to wait for a few years. Keep it watered for the first couple of years, and if possible keep the roots cool.

Hi I grow an orange flowered Honeysuckle (Which flowers in June) between a Ville de Lyon clematis and a John Davis Rose side by side. The rose and the honeysuckle flower on the same time and when the latter peters out, Ville de Lyon starts flowering. Here is a photo of the the rose and the Honeysuckle. The clem is not in the photo.

It all depends if you want to have flowers at the same time or not. If, I am not mistaken Serotina is a mid summer flowering vine, so you can plant a group 2 clematis, which flowers in June and September.
However, if you want to go for uniformity then I second, Dzitmoidonc. It will take 2-3 years until the vines are established.
Another solution is planting a couple of "Blue Moon" Wisterias. These American varieties are not as uncontrollable as the Asian ones, are fragrant and are hardy in your zone.

Here is a link that might be useful: Blue Moon Wisteria thread & photos