When investors open a managed account, they give a portfolio manager complete authority to trade or make buying and selling decisions in the account on behalf of the customer.
For that reason, clients cannot place trades in the same account that the money manager is trading. Doing so, would interfere with the manager's operations and result in total chaos.
If the client wants to trade himself, one thing he can do is open a new, self-traded account. If instead the customer is dissatisfied with the manager or the trading results in any way, he could revoke the manager's Limited Power of Attorney (LPOA) over the account (read this page for more information on LPOAs). Once the LPOA is revoked, the manager no longer has access to the account and the investor can take over the account management completely.