Deed of Trust - Main residence rule

boyrobot

My wife and I live in the property that we have been living in since shortly after we got married 6 years ago. 
My wife has never been added to the mortgage and I never got round to adding her to the title. A few years ago, she purchased a small investment property in her home country so effectively, as a couple, we own two properties but live in our main residence. 

I would like her to be added as an owner but for various reasons, we would prefer she did not take on the mortgage. Can this be done through a deed of trust? 

My understanding is that if we were to move out and purchase a larger property, this would then be seen, from a stamp duty perspective, as a replacement of our main residence i.e. my adding her as an owner, this would remove doubt around main residence and ownership. Is that correct? 
Our mortgage lender is Natwest. Does Natwest usually allow ownership of a property to be updated to reflect 2 owners (who in this case, happen to be spouses) without the 2nd spouse taking on any of the mortgage debt? 

Keep_pedalling

As a married couple you can only have a single main residence so there should be no possible confusion about which house it is.

boyrobot

This is the main residence/second home rules relative to SDLT.
The SDLT rules clearly indicate that the extra 3% rate does not need to be paid only if
1) the purchase is to acquire the purchaser's first property or
2) to replace the main residence that is owned. 
My wife does not own this main residence and is not a first time buyer, hence these questions.