The F-15 and F-16 used the same engine. As did later variants of the F-14.I thought those were designed for F-35? Would you not need a different engine design for a dual engine design over single engine?
The J-16 and J-10 also use the same engine.
There were proposals at one point to make a F135 powered twinjet. I think this was called the FB-22.
If more aircraft used the engine it would improve its economics.