All of the Mega's UARTs work full duplex.
The Mega has to receive a message before it can transmit it to something else so it can't be simultaneous.
If you want several devices to receive the message at exactly the same time then you need to connect the master's Tx line to the Rx of all the slaves.
You can also connect several slave Tx lines to the master Rx if you use diodes to isolate each Tx from the others. But that arrangement can only work full-duplex for one slave at any one time. The master has to indicate to a slave when it is appropriate for it to talk.
It will be very much easier to help you if you provide an overall description of the project you want to create.