Microphone recommendation for recording 2 channel demos

+1 to @yoyoyaya comments. With just a $500 budget you aren’t going to get any "great" mics, however, as he noted you could get a Zoom H4n Pro or one of the similar new ones for about $200 to $250 with onboard mics and use them. If you bought condenser mics like the Rode NT5 ($429) or M5 ($200) you’d need a microphone preamp to give them phantom power. Microphone preamps can be $$$. I’m not aware of any "cheap" ones but B&H might point you to one. The advantage of the Zoom H4n Pro and similar recorders is they have XLR inputs and onboard "decent" preamps.

For recording concerts and posting them to the Internet Music Archive after doing some postproduction work on the files, I record with a Zoom H4n Pro but use external mics. I have the Rode NT5 stereo pair (which are generally fine) but usually use my nephew’s older but slightly more detailed bass response AKG-391 mics.

If I were younger and had the money, I’d pop for some Schoeps MK4 and a SoundDevices recorder with great onboard preamps.

Note that these Zoom and Tascam and Sony digital recorders can do a lot more functionality than just recording two-channel audio but don’t let that phase you. You won’t need to worry about all that "other stuff". Recording is fairly easy.

I see you got a Tascam X8. I suggest the $200/pair Rode M5 as a low-cost microphone solution that should be fine for your use case.

One "issue" I have with YouTube audio demos (other than it being lossy) is that the posters very rarely (if ever) state how loud they are driving the speakers, so you don’t know if they are played at low volumes (75dB) or are being pushed to higher, more stressful volumes (90dB and above) so it makes gaining any useful knowledge from the demo questionable. Have fun.

@smodtactical you’d have to check but both Amazon and B&H offer a good 30-day return policy I think, so you could order the Rode M5 matched pair for $200 and give them a shot compared to the onboard mics (and the Yeti). Get some cheap but decent microphone shock mounts too and a "stereo T-bar", so bass doesn’t get transmitted to the mics via your stand and cause "muddy" sound or even picking up nearby footfalls on the floor.

You could send them back if you thought they weren’t a significant step up over the onboard mics.

I know a girl who used the $200 Rode M5 mics in a set up and would stream audio and video via her phone of shows done by the band Spafford on Soundcloud. Even with the digital compression going on to mp3 files or whatever they used, it sounded OK to listen to. The NT-5 mics are $429, so not sure at this point if you want to spend the additional $$$. Be careful. Recording is a rabbit hole that will draw you in and never let you go.

@smodtactical ​​​as @asvjerry mentioned, you can rent mics from Guitar Center and try several, but their selection is limited. While I like my Rode NT5 matched pair just fine, (for $429), if I had to do it over again, I'd likely have moved up to about the $1000 price point and got some Beyerdynamic MC930 Cardioid mics. Those have a long history in old forums like DATHEADS as being very linear sounding. 

Ironically, for what I do (recording live concerts) a slightly "bright" mic having a +3dB or so bump at 10,000 to 12,000 Hz can help mitigate the "hollow" sound of a large room, but the Beyerdynamic is more linear. 

A bit of history:  Mics were intentionally given a slight boost in the "presence" range long ago when recording was analog such that airiness wasn't lost, but now with digital recording, that isn't as necessary.   Enjoy recording, enjoy listening.