"My brother and I will be at the festival on Friday."
Translation:Ο αδελφός μου και εγώ θα είμαστε στο φεστιβάλ την Παρασκευή.
As Christina said, there are no other words I can think of that have this change.
About the δ-τ (άντρας-άνδρας) and β-π (κολυμπήθρα-κολυμβήθρα): In ancient Greek δ was pronounced as d and β as b. When the change to th and v occured, there was a rule that said that if δ comes after a ν and if β comes after a μ, then it is pronounced as d and b respectively. Then the rule faded out (in Medival ages) but people kept on pronouncing them as d and b in those cases. So the spelling had to change to fit the pronounciation and the new rules. Since ντ produces the nd sound and μπ the mb sound, άντρας with the new spelling rules and άνδρας with the old spelling rules would sound the same (andras), the same thing applying to κόμβος-κόμπος (kombos) for example. Because the old spelling stayed, though, people could pronounce what they see (anthras or komvos) and still be correct. So, two spellings and pronounciations are valid. In some rare cases, those two spellings came to form different words for example κόμβος meaning junction and κόμπος meaning knot.
About κ-χ, it only occurs before τ. There is also the τ-θ change that occurs after a χ sometimes (εχτρός-εχθρός) but those θ to τ changes are very very very colloquial. Those pair of changes have the same reason as the other ones, I believe (although I have not really searched the matter). It is probably because θ was something like τθ (in terms of pronounciation) in ancient Greek and χ something like κχ, or maybe it was dialectal things.
Unless I'm mistaken, no other common words have this dual spelling. As per why this happened, I think it has nothing to do with the actual accent of ancient Greek. It was a tendency during the Middle Ages to change the λφ ligature to the ρφ one. Languages change, but not always in norms. ;)
Thank you cristina. That change from "l" to "r" also exists in Spanish, for instance, but it is considered to be very colloquial, and many times linked to Southern or gypsy dialects (which are heavily used in Spanish and Portuguese slang, btw). For instance, in the South of Spain you might hear "argo" instead of "algo" (Spanish for "something").