Thanks. I had the impression you had a specific doubt about the ability to use two devices on one Uno.
It's been a few days so let me restate my understanding. I think you want to connect a HC12 and a 315MHz wireless device to a single Uno. The Uno will be receiving data on both devices and will not be transmitting on either one. Is that correct?
Because I am not familiar with those wireless devices I don't know if they buffer an incoming message or if the Arduino must pick up every single byte as it arrives. If they have a buffer it makes things much easier.
How often will messages be sent to each wireless device and how many bytes will there be in a message?
What is sending the data? Have you control over then so that you can make them send test messages?
Have you written a program to receive data with each device separately? If not that must be the first step. Write the two programs so they don't use the same resources as that will make it easier to combine the code later.
...R