Power Meter and ERG Mode during a workout

You can always disable the Assioma for that workout. No need to delete it, disable it (the toggle). That said it should not matter what meter the edge uses. In ERG mode the Edge tell the trainer what power to hold and the trainer does everything itself using its now control and power meter.   What the Edge displays is irrelevant.

Ok, but I cant turn off erg mode from Edge and this is a big issue when you have to do some drills like sprint.

as i know there is NO "powermatch" in Garmin devices so your head unit will use the smart trainer's power, the head unit can handle only 1 power source during a training. for using Assioma's power as reference you must use: a 3rd party app like Zwift or others which can do "powermatch" or as i know there is possibility in Elite myetraining app where you can add your bike powermeter to the trainer and using the assioma's power instead of the inbuilt optical torque sensor in Direto. and after this pairing in every app or in Edge the displayed Direto's power is your Assioma's power in reality (the Wahoo has a similiar workaround in the Wahoo app, it changes the power setting in firmware level)

turning on and off the ERG is an another impossible task in Edge now, 3rd party apps can do it (Zwift for sure, you can insert SIM blocks between ERG blocks)

but in ideal condition, your Assiomas should show almost the same power as your Direto, lets say target 200W on Direto is 202-204w on Assioma Duo, so somewhere between 1-2%(this difference is the drive train loss), so easier to train depending on Direto's power. ofc, if this difference is much higher , there is issue with the Direto's or the Assioma's calibration/zeroing and must be inspected where is the problem.

and one more advice: do not do sprint training indoor without rocker plate, you will damage your trainer and/or bike if you do the sprints in same way as outdoor (standing out with full body movement as sprinters do).