For example, at the ISKCON London temple, the principal dieties are Rādhā-Kṛṣṇa according to their website.
About the Deities
On the right-hand side of the shrine, you will see Radha-Krishna deities...They are the principal deities at our Temple.
On the left-hand side of the shrine, you will see Lord Jagannath (another form of Krishna on the right-hand side of this part of the shrine), Lord Baladeva (Krishna's brother, on the left-hand side) and Lady Subhadra (Krishna's sister, in between Jagannath and Baladeva)...These unusual deities are based on the original deities in Jagannatha Puri located in Orissa, India.
In the centre part of the shrine, you will see Sri Sri Gaura-Nitai - the incarnations of Krishna and Balarama - and who are the propagators of harinama sankirtan which is the process for spiritual perfection in this age.
So how does ISKCON decide which deities to install at any temple and which should be the principal deity?