RF splitters are readily available as they are typically used in labs for RF measurements. They work by diverting half the power to two ports while transmitting, and by combining incoming RF energy while receiving.
In principle, splitters can be used to increase the coverage, e.g. when you have a metallic fuselage that makes it impossible to put a single antenna with good reception. However, destructive interference can just as well achieve the opposite: The two signals coming from each antenna cancelling out.
We thus recommend not to use splitters. Instead, consider a device with true radio diversity, such as the PowerFLARM product line.
Find more information in FTD-041: Application Note FLARM Antenna Installation