Can you open a Joint ISA?
The simple answer here is no. You cannot open a joint ISA account as the tax benefits given with ISAs are individual tax benefits and it will be impossible to allocate those benefits in a case where a joint ISA account is opened and the savers have varying tax profiles or eligibility for any tax reliefs which are available through ISA products.
The only way to benefit through Joint ISAs will be to open a similar savings account where joint accounts are possible.
Most people look for joint ISAs as they believe they could share the tax reliefs with their partners, unfortunately this isn't the case and any tax relief such as the personal allowance that would have been available with a joint ISA will have only applied to one of you and would be non-transferrable if joint ISAs were to exist.