Of course it is not possible to enable more than one transmitter at a time!

Don't you think this is obvious?

You have to check if the channel is free and then transmit. Or even better have the PC to ask each of the 40 PICs to send the data. So you have to use a pair of Tx-Rx modules on every point. If this is too complicated for you then you might consider a XBee module that is transparent serial tranceiver. Up to 1 mile!

Ioannis