Just out of curiosity, is cost really an issue practically speaking? You can get a multi-channel AVR at the same, or lower price point if you go with a one year old model, so is it really cost, or more the "form follows function" principle you want to stick to?
For example, you can get the NR1608 now for only $479 or the NR1609 $499, brand new.
https://www.amazon.com/Marantz-NR1609-50W-Amplifier-Compatible-Bluetooth/dp/B07CVGRMX2
or $699 for the AVR-X3500H if size is not an issue.
Aside from the from follows function principle, the two channel versions offers no practical advantage that I can think of. The fact is, inside the box of those two channel units you will find the same kind of electronics such as ICs, DACs preamp/amp section used in the 7.1 units, except few amps on the boards and the 7.1 ICs are connected for 2 channel use, i.e. not fully utilized.
I suppose there is one advantage, that is, less parts to fail and less heat, that could be easily and virtually equalized with the use of an external quiet cooling fan.