Acer and Asus make dual SIM phones. One SIM is 'regular size and the other is micro-SIM for the Acer. I have been running the Acer Liquid Z220for a few months and only have concerns about the battery drain. Both SIM slots support HPSA+. My SO uses an Acer Z220, using data on one network and voice on the other. You can set up the Acer to ask which SIM to use for voice, messaging and data, before sending communications, or set defaults. It is that easy.
I have a friend who got an Asus Zen Phone, but did not ask him about it.
Then, I think that the Asus and the Acer are very similar. Did the Asus run Android Lollipop?I got the Zen Phone when I was traveling a lot in Asia and needed a local number before Bell finally got decent roaming rates. The way this phone works is that it tells you when a call is coming in on SIM 1 or SIM 2 and both SIMs are located on the very easily open-able back. Very decent prices for this phone, far below Apple or Samsung prices. Not as flashy looking. If you are looking for a second line for burner purposes, however, I'd keep your numbers far apart (ie. different phones). I'd never be 100% positive that the two numbers don't share information on the one phone, ie. call history, search history.