Is it possible to filter radio-waves using another radio telescope? When making an observation using a radio telescope, is it possible to filter out noise coming out from a known source by using another telescope that will focus on that noise source?
By filtering, I mean processing the observation on a computer and calculating the filtered image using the second telescope source. Assume that both of the telescopes are connected and can easily sync on time and location and exchange other information.
Coherent interfering signals are usually referred to as RFI (or Radio Frequency Interference).  One could certainly digitally filter out a coherent interfering signal from a telescope by observing from an additional telescope focused on the source of the interfering signal, given the proper geometry.  Here is a research paper on Radio Frequency Interference Mitigation that outlines some of these very techniques in section 3.4: Post correlation studies.
Incoherent interference is typically referred to as noise, and won't be the same at the two telescopes. This means an astronomer can't use one telescope to filter out the noise at the other. Some examples of noise sources are:

Noise that comes from thermal from the telescope's electronics
Ground backscatter into the antenna receiver
Inaccuracies introduced by analog to digital conversion and other signal processing.