Can I issue a section 21 notice at the same time as issuing a new fixed term AST?

Possession - Section 8 and Section 21

Can I issue a Section 21 notice (with 6 months notice) at the same time as issuing a new 6 mth AST? This is a follow on to my earlier question - the background is that one of our tenants is separating from his wife - he wants to leave - his wife wants to stay. We are considering issuing his wife with a new 6mth AST but would like to also issue a Section 21 at the same time as we are not sure whether the wife will be able to manage the property on her own and if she can't could the Section 21 then kick in at the end of the 6mth AST enabling us to go straight to accelerated possession? Or would we have to wait until the fixed term AST comes to an end before issuing a Section 21? Thank you