Even this is illegal, clients ask for dedicated 8 hours which is not happening over here. If it is not in contract by client then their legal team is not strong enough.
While switching projects, I was asked not to mention that I am still supporting old client / new client to each of them because of contractual issues (old project was not ready to leave me right away while new project had to be kickstarted)
Instead, I was provided with a resource to help till the time I could complete the transition from the old project (~1 month).
We also have people across multiple projects officially and clients know about them and clients are charged according to time devoted by the employee to them. Usually, 50-50.