The amalgamation of two or more Parishes into a single one is an action that has to be carried out on application through the local authority as a Parish is a designated statutory body. You just cannot bring two Parishes together by agreement between them regardless of how practical it is to the Parishes.
Once a new Parish is designated and formed via the LA then they act as one and the annual parish meeting, for the new parish, has to be organised and held by the new Parish Council.