NEC 400.7(A) Flexible Cords point 11 would seem to allow it so long as it was 1-to-1 inlet to outlet.
Flexible cords and cables shall be used only for the following...(11) Between an existing receptacle outlet and an inlet, where the inlet provides power to an additional single receptacle outlet. The wiring interconnecting [...] shall be a Chapter 3 wiring method. [...]
I don't know what Chapter 3 is though, and in the below study guide link it seems to show a double outlet in the graphic.
thats old code, nec is now 2020. i dont have an nec codebook...but pretty sure you cant use extension cord connected to the ups connected to another receptacle...the section of the relevant code would be labeled something like 'uses not permitted.'
the code you referenced states 'Between an existing receptacle outlet and an inlet'
the circuit is also unlikely to be arc fault protected, which would mostly likely be required.
Flexible cords and cables shall be used only for the following...(11) Between an existing receptacle outlet and an inlet, where the inlet provides power to an additional single receptacle outlet. The wiring interconnecting [...] shall be a Chapter 3 wiring method. [...]
I don't know what Chapter 3 is though, and in the below study guide link it seems to show a double outlet in the graphic.
https://www.electricallicenserenewal.com/Electrical-Continui...