You will need to use an external device one way or another if you want to either the internal mic or your headphone mic. Though, for decent recording I don't recommend either as they are (barely) designed for the most basic recording tasks.
The problem lies with the headphone jack. There's a physical switch inside the port itself that disconnects the internal mic so, if you choose to use that port, the internal mic will get disabled.
If you want to aggregate two mics, here are some solutions:
- Get two USB mics. Any decent quality USB microphone will do. You can then aggregate both into a single input.
- Use a USB DAC. This is like a USB sound card for your laptop with both input and output. You can use a mic with with a phono plug to a 2.5mm plug with the right RCA adapter which are very inexpensive.
- Use a dedicated USB mixer. Personally, I wouldn't use anything "built-in" and instead opt for a mixer that allowed me to fine tune the gain on individual microphones.
I personally use the second option mostly as a USB DAC for connecting my Bose Wave radio to my iMac for quality sound, though I do use it on occasion when I want high quality recordings (either via microphone or line input)