Receiver and transmitter with the same antenna
A typical 434 MHz low cost transmission system will have a free-space path loss of: -

Loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d) (Friis equation in dB form)

Where f is in MHz and d is in kilometres.

Transmission distance will be about 0.1 km and the pathloss works out at: - 

32.5 dB + 52.8 dB - 20 dB = 65 dB

But antennas will provide some gain (about 2 dB each end) so the free space figure is more like 61 dB. However, most RF engineers will add another 30 dB for fade margin and this means the overall path loss is about 90 dB.

Using a splitter at both ends will degrade the power transmitted by 3 dB and degrade the power received by 3 dB; a total of 6 dB.

You then have to ask yourself if that is acceptable or not. For most cases, simplicity overrides performance and it isn't a big deal. However, 6 dB is equivalent to halving the range from 100 m to 50 m. I can't tell you if this is good or bad.