I am also very interested in the topic and eager to get informations about actual "real" tests that are going on. According to this Spanish article there were tests with Bitcoin, Zcash, Litecoin, Decred, ARK and Vertcoin, some weeks ago.

The traditional atomic swap model supposes one multisig transaction per chain. That obviously isn't enough for high frequency trading but would enable the typical use case of a trader that wants to "ride" bullish and bearish cycles of distinct coins. For higher frequencies, one could use LN, or maybe even a specialized pegged sidechain in the future (RSK?). For LN, however, I think it would be used mainly for small exchanges of less than 50-100 USD because of the slightly increased risk.

By the way: There is a atomic swap system that works for two years now used only by two small altcoins but supposedly could be adapted to Bitcoin-based coins. I have recently downloaded both blockchains to test it soon, if I am successful I'll report results here.