How can I operate two ASK RF Transmitter modules(433 MHz) simultaneously without jamming each other?
Your receiver is fixed to a single frequency and can only receive the signal from one transmitter at a time, so your only option is some kind of time division multiplexing (ie. each transmitter sends a burst in a different 'time slot' when the other one is idle). 

Problem is how to make sure that each transmitter stays in its own time slot and not stomp over the other one? You need some way to synchronize them. One way might be for one transmitter to include a receiver which listens for transmissions from the other one, then it can avoid the other transmitter's  time slot. Or you could occasionally send a synchronizing signal to both transmitters, which then stay in sync long enough to avoid colliding in the mean time (how long would depend on how accurate the clocks are in each transmitter).

Alternatively you could just let them collide and rely on the decoder to sort it out. For this to work reliably each transmitter needs to send bursts at a different (asynchronous) rate or with random delays. Occasionally they will collide and then the corrupted signal will be rejected by the decoder, but mostly they will send at different times so their signals will get through. Since RF transmissions can be corrupted by noise and interfering signals anyway, this is not much different from using a single transmitter (just a few more lost data packets).  

The HT12E takes about 40ms to send a burst, so the absolute maximum rate for receiving two transmissions is about 12 per second. However if the transmitters are not synchronized then you may need a much lower rate to reduce collisions. If that is not acceptable then you might have to look at using more sophisticated transceivers.