designing trace anthtenas for 2 sets of rf rx+ tx
I think it all depends on what range / sensitivity you expect to achieve. What I mean is: combining antennas will introduce some losses and that will decrease the sensitivity of the receivers and it will decrease the power the transmitters can transmit.

A one antenna solution is possible for sure with the restriction that you do not send and receive at the same time at the same frequency. If you do not do this then I see no problem.

You're in luck that such 315/433 MHz modules are not so "properly RF designed" and you can very likely get away with a simple way of combining all the antennas. The length of the antenna should ideally also be different for 315 and 433 MHz but that doesn't mean that a 315 MHz antenna does not work at 433 MHz. It does work, it is just a little bit less sensitive.

What I suggest you try is the following, to prevent the modules damaging each others input/output I would use small capacitor in series with each input and output. A 1 nF capacitor will do I think. So: get four 1 nF capacitors and one antenna. Connect all inputs and outputs to the antenna through a 1 nF capacitor. I must emphasize that this is not the ideal solution for optimum sensitivity and range but it is cheap to try and maybe it does the job.