How can I use the power output of my power meter on my bike instead of the power output of the Taxc Neo 2T?

i dont know what kind of powermeter do you have but the Neo is more accurate and reliable than 99% of bike power meters.... if the "problem" is that your bike powermeter is showing higher watts , that can be normal if it is 1-2% higher number (so 1-2-3watts differences at average cycling Joe's power numbers...), if more , there is a high chance that the bike powermeter has bad measuring (tons of crp out there....).

answering for the quiestion, where do you wanna change the power source? in Zwift You can easily do it, in MyWhoosh too, the power source must be the bike powermeter , and the trainer is the controllable. 

This may be true however it's worth factoring in drive train power loss between a crank/pedal based pm & hub based pm ie the trainer. I'd go with my speedplay pedals everytime which consistently read neo pwr + 5w, which can easily be lost in the drivetrain & has more accurate metrics available too

yes, due to the drivetrain loss there is that tiny diff what i mentioned (depending on used gear ratio, i meant for small ring front middle rear in ERG, on big ring front and on 13-14T rear it is much bigger diff, but it is rarely used during an indoor session, only on fast downhills in this virtual apps). the 5watt can be good or bad too, if i know good, you cant valide and re-calibrate the pedal's factory calibration (so the real 200watt is 200 (100-100)watt by the pedals, or measured 202 (100-102 if the right side is a bit miscalibrated) or 204 (102-102, both pedals equally miscalibrated with 2% ) with static weight test (hanging 20000-30000gramm weight on one pedal and "ask" the pedal how much gramm is the hanged weight). and this procedure is mandatory imho on all pedal-based powermeters (Assiomas knows it, Garmins too), because the factory calibrations are not an ultraprecise calibrations... (or not done). the Neo is calibrated with a dynamic torque sensor device up to 800-1000w, so that is ultraaccurate (ok, sometimes somebody makes mistake in the factory, i met already with faulty Neo2T which heavily overmeasueed the power with 15%)

a single sided pm like Stages, 4iiii, InPeak also not good , but the dual sided neither (the Shimano's pm is the worst of all), these are not accurate powermeters. only the latest InfoCrank dual is good (because it has symmetric cranks and using latest strain gauge techincs), the spider-based Power2Max NG is the reference since almost 10 years, the dual Assiomas (old model and new PRO) also good pms, the rest are just below it by accuracy, precision and reliablity. i am always riding 2 powermeters on my bike (now with P2M NG and Assioma PRO RS2) with dual measurements and with triple recordings on different Neos. but i am doing it since 13 years (with different powermeters)

I use a power2max NG.

I ride training sessions from my Edge 1040, but so far I haven't been able to find where I can set the power meter as the source.

Is it normal for a low speed to be displayed?

At 300 watts, it's 21 km/h. With Kickr Core, it's 36 km/h at 300 watts.

if You train in ERG mode, than the 21km/h with 90CAD is normal if You ride with small (34 or 36 front) - 19T rear "fixed" gear. You can ride 21km/h with 50watt and with 500watt also with the same CAD. but if it is too low for You, You can modify the wheel circumference for ~3000mm (instead of 2105 or similar) , immediately You will have 1.5x higher speed. 

You can set the NG as displayed power in Edge (paired as power sensor via ANT+), but the 2T (as FE-C equipment) will be the "master" and its power will be the reference for the workout, i mean, if You set in the Edge workout 10min 200W, then the 2T will set it by its own 200watt, on the displayed power value datafields You will see the NG's power values, if You are lucky, it will be 1-2-3 watts higher number only (at this resistance level). and this NG power values will be stored in the FIT file. if i am right, it is working like that (since a while), but if You will do a training like this, send/share me the FIT file and i will tell it after analysis.