Transmitting using this transmitter seems to jam many wireless devices (possibly using the same frequency) like car keyless entry remotes, garage door openers etc. Moreover, two such transmitters cannot be used simultaneously as they jam each other and the receiver gets no signal from either. One workaround is to use an oscillator to transmit discountinously (a short burst every few seconds). But I was wondering if there is a way to solve the jamming issue and transmit signals from both the transmitters simultaneously. The receiver is common for both the transmitters (assigned different addresses using HT12E) and hence signal from just one transmitter is used at any given point of time. Data sheet(not of the exact product but is similar) https://electrosome.com/wp-content/uploads/2014/10/MO_SAWR.pdf
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.
You need to find a medium access control mechanism that will suit your needs.
The easiest is to designate one device as a master and the two others as slaves. The master will ask only one of them for data at a time. The slave will only transmit if asked by the master, so you can easily avoid collisions.