Your understanding of a struct, its definition and usage are flawed.
void loop() {
struct Data {
float temp = bme.readTemperature();
float pres = { bme.readPressure() / 100.0F };
float alt = { bme.readAltitude(SEALEVELPRESSURE_HPA) };
float humid = { bme.readHumidity() };
} allData;
radio.write(&allData, sizeof(allData));
}
I would put the definition of the structure into a .h file, to be used in both sketches.