bhai, these days, there is no compulsion to continue with a broker, if the service is not satisfactory.
One can open trading account with multiple brokers
Scalpert and NEST can be used together but giving other application is not possible with any brokers. All trading App work on RMS and OMS system. unfortunately, there is no provision between BEST, NOW etc to update live trades. So, if we place an order on NOW, ther is no option to syn with other application to give us correct MTM. So, my understanding is that no two RMS/OMS application shall be enabled together for risk purpose.