Unfortunately there isn't a product that could preform this exact ability. However, there are 2 methods to get this to work
1. If the 3.5mm monitor output jack from your microphone is at line level, you could possibly hook it up with a 3.5 to RCA cable and connect it to a DAC/AMP that has RCA and USB input. xDuoo TA 01, has both RCA and USB, you could hear both sources once headphones are connected.
2. Get a schiit sys. This would allow 2 input into 1 output with 0 latency, but it only has RCA inputs which means you will need a external dac for your computer source. Furthermore, you will need a external amp for the RCA output. (This method is not recommended)