Make sure you work out your figures with no interest allowance on the rental properties, as it was reduced in the last budget and may be reduced further.
Make sure you can afford the new house with no mortgage interest relief for the same reason.
As it is, if you do equity release on them, you will only be entitled to claim the interest on the portion that was already mortgaged, not on the equity release bit aswell.
Whereabouts are the properties? What're the local rental markets like? Have you told the banks you are renting out the first property? (Commercial mortgage rates apply to residental investments - they are higher than residential rates). Have you factored in insurance and maintenance?