I've seen discussions about the minimum amount of stock D76 for one roll of 135 or one roll of 120 hashed over many times here and elsewhere. It's still unclear to me.
The Kodak datasheet says
Quote Originally Posted by Kodak
If you use D-76 Developer diluted 1:1, dilute it just before
you use it, and discard it after processing the batch of film ...
You can develop one 135-3 roll (80 square inches)
in 473 mL (16 ounces) or two rolls together in 946 mL
(one quart) of diluted developer. If you process one
135-36 roll in a 237 mL (8-ounce) tank or two 135-36 rolls
in a 473 mL (16-ounce) tank, increase the development time
by 10 percent
One way of interpreting this is that the minimum amount of stock to be used for a single roll is ~250ml and so 1+1 requires a total volume of ~500ml including the diluent.

In other places I've seen different interpretations, and I've seen other folk say 100ml stock is enough for a roll.