If you are on a Mac, look up and read about Core Audio. I believe the OS provides ways to aggregate devices into a single driver. You’ll want to have both devices share a common clock if at all possible. Perhaps over SPDIF, wordclock, ADAT, etc.
With Windows, using multiple devices requires special drivers and some way to have the audio devices share a common clock (Word Clock, SPDIF, ADAT, etc). I.E. Some products out there can run multiple devices of specific makes and models and present them all to a DAW in a way that appears to be a single device. I.E. I run a pair of Delta 1010 cards that share a common clock with yet a 3rd SPDIF device that connects to one of the Delta 1010s. Through SPDIF and wordclock, I can make sure all of the devices lock onto the same clock. M-Audio shipped the Delta 1010 series with special drivers that aggregate all compatible Delta models into a single ASIO driver.
If the devices support some kind of clock sharing protocol, but don’t come with compatible drivers or backend to aggregate the devices you could try ASIO4ALL (Windows Only).
ASIO4ALL sets up a backend to convert WDM drivers into something that ASIO hosts can detect and use. In the Advanced settings of ASIO4ALL, there are ways to aggregate multiple devices.
Is it good? That’s all relative to be honest. It depends on the device and the quality of whatever ASIO drivers it ships with. For some of the lower or mid range devices out there, ASIO4ALL along with bog standard Windows Reference drivers is as good or better than the stuff that came with the device; however, for most of the higher end interfaces, it doesn’t come close (latency/stability/etc).
With ASIO4ALL it is possible to aggregate devices that use independent clocks, but be aware you’ll get drift between the two signals. Sometimes that doesn’t matter much (I.E. Temporarily aggregating an INPUT like a USB mic or something), but in others, the longer the project is on the time line, the more it matters, and the clock drift can indeed become ‘noticeable’. You wouldn’t want to do any precision mixes or anything without somehow syncing up the devices.
As for headphone jacks on Laptops…
Those can be pretty weird under any OS. Sometimes they have all sorts of switching logic in them. I.E. Disable if nothing is ‘plugged in’, and instead route audio to internal speakers. Do something yet different if a mono plug is connected, yet something else if a stereo ring-tip sleeve plug is detected, and yet more if a plug has 3 or more rings. It’s also possible that laptops might do strange different things if there is also some kind of BlueTooth audio introduced into the audio matrix. Some might even have sensors to detect signal load/impedance/etc, and change all sorts of things that might or might not show up and work with a given DAW. So, for pro audio, it’s typically best to avoid trying to use built in headphone jacks period. Use a single external audio device, or multiple devices that are ‘designed’ to work together and come with the required drivers to aggregate them.
