Well, the bottom-line is to prevent any contaminations. If you are targeting RNA it will takes time to extract RNA so there must be no room for errors.

Christian Fernando Montoya Estupiñan

When doing DNA extractions we often use RNase to digest any RNA that is in the sample and DNase free water. When extracting RNA we do the opposite. It is effectively to prevent extractions for RNA specific assays being contaminated with DNA and in DNA assays contamination with RNA. Effectively you want to keep your yielded extracts as pure as possible.

Theoretically you could do both in the same area, you just need to keep everything clearly labelled and make sure you are using the right reagents. Most people would recommend you use filter-barrier tips with RNA extractions that you keep separate from you DNA extraction tips.

A main concern may also be the temperature... DNA is reasonably stable at room temperature while RNA degrades more easily.

depends on your workflows, and downstream analysis. if some assays are ultra sensitive and tricky (eg you are trying to detect extremely low levels of donor derived cfDNA for post transplant monitoring) - you want to make sure there is no contamination originating from nucleic acid preparation. (you can even implement wipe tests, in addition to separating rooms and equipment) If there is nothing super sensitive- its totally fine to purify RNA and DNA in the same area