According to coindesk article, Ethereum foundation will proceed with the Casper and sharding update at one go. Casper was a sidechain implementation and was being tested in the form of a smart contract named EIP 1011 and was supposed to be implemented first.It was supposed to shift Ethereum from proof of work to proof of stake algorithm. Sharding which was a more complex project, which involved splitting the blockchain and storing it into different servers, was to come next. Recent advances in the blockchain tech have made it feasible to implement both upgrades at the same time. According to buterin if it does get implemented, then it would combine to create a version of the protocol that could be orders of magnitudes more efficient.
The advantage of implementing the tech at the same time would be that instead of requiring 1500 ETH for staking it would only require 32 ETH and thus making the blockchain more decentralised.
According to Sharding developer Justin Drake the implementation will also have a security advantage as this system would allow for an “animosity” between nodes on Casper and sharding, increasing the security of both systems. “You can’t be a Casper validator without also being a sharding validator,” Drake said.