Section 21 Notice already served on both tenants - one has already left.

Possession - Section 8 and Section 21

Is the Section 21 notice served on 18 December 2013 still valid as I served it on both tenants named on the tenancy agreement and have since been informed that one of them vacated last Sept [but did not advise me]. They are now in arrears - can I also serve a Section 8? If so, does it have to be two months from today served on both tenants or just the one remaining tenant?
Thank you.