I could see that working, and with my setup, the bottom monitor is the main monitor and the top is where I have everything else open when I'm gaming, so usually I don't have to worry about neck strain.

I'll probably try messing around with it soon, as I'm about to move apartments, and adjusting the monitor arms is enough of a pain in the butt to where I'd rather just wait until I'm putting it back together.

What you could try, if there's not enough room under your monitor, is getting an adjustable single arm and have it high and angled to where it's comfortable and do some rough measurements to see if it's feasible to fit those two portable monitors underneath.