Topping off conventional coolant with long life coolant

I have heard that if the cooling system in an older vehicle that has conventional coolant is topped off with long life coolant it will shorten the life of the conventional coolant. Is this so?

No, because the normal life of conventional coolant is only 2 years/24000miles, adding long life coolant will have no effect on the service life of the conventional coolant. However, adding conventional coolant to a cooling system filled with long life coolant will shorten the life of the long life coolant.