You've basically answered your own question when you wrote "they are meant for married couples who want to leave everything to each other".
The term 'mirror will' has no legal meaning but it's used when Fred's will says "I leave everything to Mary" and Mary's will say "I leave everything to Fred". Of course the wills can actually be a little more complex than that, saying (for example) "I leave half of any monetary assets owned by me to be shared equally by my children and the other half, plus my share of the house to Mary/Fred".
(Before anyone points it out, such wordings are legally full of holes. I'm simplifying things here).
If a couple own their home as "joint tenants' then it's their legal partnership that owns the property. When one partner dies, the other partner automatically owns the whole of the property. (Even if the deceased person left a will stating "I leave my share of the house to Battersea Dogs' Home" that part of their will would be invalid. The full ownership of the property would still go the surviving partner).
However if a couple own their home as "tenants in common" they each own a particular share (normally half) of the house. They can leave that share to whomever they wish (including the Battersea Dogs' Home).
Similarly a married person (or a person in a civil partnership) can leave any other part of their estate to whomever they like if they write a will.
If a married person (or someone in a civil partnership) dies without making a will (i.e. 'intestate') part of their estate goes to the surviving partner, with the remainder being split between their partner and other relatives (such as the children or siblings of the deceased person, depending upon the circumstances). Until recently (1st February 2009) the first £125,000 of the deceased person's estate went to their partner, with the rest of