Considering that both (https://www.fanfiction.net/Charlie-s-Angels-and-Hart-to-Hart-Crossovers/117/5879/) were produced by Aaron Spelling and aired on ABC concurrently. Hart to Hart's first season (1979-80), would've ran alongside the fourth season of Charlie's Angels. Not only that, but Robert Wagner actually had a financial stake (https://www.hollywoodreporter.com/business/business-news/court-denies-wagners-angels-claim-127703/) in Charlie's Angels (https://fycharliesangels.tumblr.com/post/18293743564/in-1976-robert-wagner-and-wife-natalie-wood-were) and its creation. He was actually going to play (https://screenrant.com/charlies-angels-robert-wagner/) Charlie Townsend in the ill-fated 2011 reboot, but for whatever the reasons, the deal fell through.

I would rather have seen a crossover with Starsky and Hutch.