RF splitters are typically used in labs for RF measurements. They work by diverting half the power to each of two ports while transmitting, and by combining incoming RF energy while receiving.
In theory, splitters could be used to increase the coverage angle, e.g. when you have a metallic fuselage that makes it impossible to put a single antenna with good reception. However, in practice, destructive interference will normally achieve the opposite: The two signals coming from each antenna will cancel themselves out.
Therefore, it's not permissible 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