The tax question depends on which tax bracket you both fall into at the moment and how much profit you're expecting to make on the BTL property. If one of you is a higher rate tax payer and the other will remain a basic rate taxpayer after allowing for the BTL profit then the best solution taxwise is to put the BTL in the name of the person who will only pay basic rate tax.

For tax purposes it doesn't really matter how you structure your first property unless you've got lots of assets and are likley to fall into inheritance tax.

You'll need a reasonable amount of savings if you want to buy two properties as BTL mortgages will only be 60 - 75 % LTV and you'll also need rental income of 125% of the interest element of the mortgage payments. BTL mortgages are usually based on the rental income rather than your salary as they are seen as a commercial decision which should be capable of making a profit.