It is not recommended to use more than one Security software suite on the same computer, as this software will try to access the system resources at same time and that will decrease the system performance which may lead to a system crash situation. Also, the real time scanner of one anti-virus software may detect the other software signature files as malware and vice-versa which will eventually make your computer unsecured.
It is highly recommended to have only one Security software suite installed on your computer.
Note: Prior to doing a scan using any free security software or online scanner make sure you disable the current Security software completely.