Can I keep 2 teacup stingrays and 2 peacock bass in a 135 gallon aquarium?
Not on the bass. The rays could survive in the tank, but not optimally. The problem is the 135 gallons are only about 18 inches front to back. The Teacup gets at lease 15 inches in diameter (not counting the tail) with some getting larger than that, with the tail length totaling up around 30 inches. The bass gets up to about 20-30 inches! As an adult it could not even turn around.
Note: The rays are not Motoro rays. They are a different species, Potamotrygon Reticulata. The Motoro is Potamotrygon Motoro.
The 135 could house two juvenile rays for some time, but a wider tank is needed for the sub adults to adults. You want a minimum footprint of the tank being about 72" by 30" or more. The 135 gallon is 72" by 18". A 180 gallon is better being 25 inches front to back. A 300 gallon is one of the better tanks to get as a standard is 30 inches front to back.
So, the tank is just too narrow front to back to optimally house either fish for life.