molecular biology - Do only one or both pairs of homologous chromatids exchange genetic material during the process of crossing over?

To be specific: Assume chromosomes A and B are homologous. They've both replicated into A1, A2 and B1, B2 and have formed a tetrad at the equator (synapsis). Most textbooks show either A1 and B1 OR A2 and B2 exchanging genetic material, but I've never seen an experiment/paper proving that this is actually the case, and now I've come across this figure showing both A1, B1 and A2, B2 crossing over https://web.archive.org/web/20160314020121/http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm see ("crossing over" under meiosis)

So which version is correct, or do we simply not know for sure? Thanks in advance.