You definitely have to speak to them both but make it clear that they are both liable for paying the rent. If one of them leaves they are still legally bound. If it is a 12 month TA then one person only cannot serve notice to quit. You have to all try to come to an agreement moving forward. If you are happy for either of the Tenants to find a replacement then proceed cautiously and definately have a credit check done, personal references and perhaps a guarantor. I nearly always have a guarantor as it is added security and think that either tenant may think carefully before doing a bunk.
If you cannot come to a resolution then I believe that you would have to serve a section 21 on both of them. Or ask them both to jointly serve notice to quit. ( and/ or sign a deed of surrender) Communication is key.Make it clear to them that this needs addressing and is only likely to get worse for all parties if not.
If you agree to release one of the tenants then if believe you could ask them to sign a deed of surrender. But I don’t think you should do this until you are very confident that the Tenant remaining is aware of their obligations, you have a new tenant signed up etc. When the deed of surrender is signed for termination on a set date then after that date the tenant leaving has no legal obligations( save for rent payable up to the date of exiting)
Hope that helps
I hope that helps.