Has there ever existed a biplane fitted with a jet engine?

I think John Klatt's "Screamin Sasquatch" is equipped with both a radial and a jet engine.

http://johnklattairshows.com/

Indeed there has. The PZL M-15 Belphegor was designed in response to a Soviet requirement for a new aircraft for use in the agricultural sector, to replace the aging Antonov An-2 biplane. Polish company WSK PZL-Mielec intentionally based the Belphegor on the An-2, including its biplane wings, in an attempt to replicate its success. Powered by a single Ivchenko-Progress AI-25 turbofan that provided 14.7 kilo-Newtons of thrust, the Belphegor turned out to be a failure. Its biggest drawbacks were high operating costs and poor handling. As it offered nothing in the way of improvement over the An-2, the An-2 remained in production, whilst just 175 Belphegors were produced.

https://en.wikipedia.org/wiki/PZL_M-15_Belphegor

There are several examples of twin engine jet biplanes here is a youtube video of one

This is a Waco Biplane with a Pratt & Whitney 985 radial engine along with a General Electric CJ610 engine. This aircraft has a thrust to weight ratio of 1:1. It can fly vertical! Amazing sound of the radial combined with a jet engine.

https://youtu.be/eqMvXE4sxpk