This is because while you mine, you are using your PC’s power. All miners are designed to use all of your GPU/CPU’s power.
You also need somewhere to install the dag files, so if you’re mining for example with a GTX 1070 8GB, Nicehash’s ethereum miner will use 4.2GB+ of VRAM, leaving 3.8, which is not enough for Salad to start mining with phoenixminer, so it will try run XMRig, and in the end you will get barely any earnings both on Salad and nicehash.
TLDR: Please only use one miner per CPU or GPU
What he said ^
I tried both miners at once and all I got was a nice 0 earnings on both.