Names on tenancy agreement

Tenancy Types and Management

I let my property to a couple, it is only the lady's name on the tenancy agreement (they are not married), but usually the male tenant pays the rent each month. He doesn't pay by direct debit but instead deposits the rent in my bank account manually - very frequently he travels and the rent ends up being late. I frequently have to chase them for rent at the end of each month, it does however get paid after my reminder.

In response to a request today for such, the lady tenant has requested I send her a new tenancy agreement with her partner's name included on it, so that they are joint tenants.

Do you foresee any problem with doing this? I do not trust them 100% as they are always so late with their rent and I don't want to do anything that may jeopardise myself in any way.

Thank you for your help,

Kind regards
Amanda