YES, you can use two radio receivers / SDR radios with one antenna. The loss associated with doing this is minimal. One will likely not notice a difference without test equipment.

Using some wire and alligator clips, I created an extremely primitive splitter that connected two SDR radios to one antenna. I switched on a signal generator and noted the signal reading. I then removed the primitive splitter and connected the single SDR radio to the antenna, while noting the signal reading. The difference between the two setups was 4.4dBm.

I purchased a 'SMA Female To 2x SMA Male Y Splitter Cable'. ($9 / Free Shipping)



I performed the same test described above with the high quality SMA splitter. This time the difference was 3.1dBm.

What is 3dB? There are 6db in one S-unit. Therefor a loss of 3dB is a loss of one half of a single S-unit. This is quite trivial when you have a resonant antenna with a -105dB noise floor, as the difference in performance is less than 3%.
In the SDR radio hobby, one can easily gain 1-33dB of receiver gain by using a higher quality USB cable. One stands to gain more than 3dB by raising the antenna a few feet. If the coax from your receiver to your antenna is 100 feet, you can obtain a 2dB gain increase by using higher quality (low loss) coaxial cable. Moving your SDR radio away from your computer and in to a sweet spot can provide a 1dB+ increase in gain.

It's interesting to point out, when I run the same experiment by only watching the S-meter, the S-meter shows a loss of one quarter of one S-unit. I have to watch carefully to notice the change in signal. It's barely noticeable.

There is distant CB radio station on Live CB Radio that is on the air every day. His signal in to the receiver has always been so weak, that I could only hear his voice in the noise level. Now that two receivers are attached to the same antenna, he still sounds the same as he always did. With that stated, this loss of 3dB is not affecting the weakest signal.

In the event that you will not tolerate this 3dB loss, simply install an LNA, (low noise amplifier) between the splitter and antenna feed-line. You can get more than 3dB back, with the option to make the noise floor lower than it was before. Equally as impressive, a decent LNA can be had for the price of a fast food meal.
I was skeptical of LNA operation at first, simply because the classic signal pre-amplifier did nothing but amplify the static level with the desired signals. This is not the case with the modern LNA, hence the name Low Noise Amplifier.

The benefits associated with using two receivers with one antenna, far outweighs a 3dB loss of performance with both receivers. Setting up another antenna and feed-line can be a an expensive project, while a loss of 3dB is trivial.

Make note, this primitive splitter is mainly an HF / VHF Low concept. The losses will increase substantially at higher frequencies. For VHF / UHF / higher frequency applications one should look in to a 'multicoupler' device. However, if you only monitor stronger signal VHF / UHF and higher radio traffic, the loss may still be trivial.