So if you found yourself locked out due to att disabling 2g in their network (in the US) and now you find that your dual Sim phone can only have one Sim working at a time, you may be looking for a solution, or a phone that can enable both Sims simultaneously again.
So you probably have found the one plus one and wonder if this will serve for the purpose of having dual lte Sim enabled at the same time.
The short answer is yes :), one plus 5 works with dual att Sims, go for it.
The long answer may start with, why is my current phone not able to work with dual lte Sims, what is the point of dual Sims then, or why did the manufacturer of the phone, in my case one plus, build it without the capability of dual lte. The answer is actually, is not a mistake from the phone manufacturer, but rather from the soc manufacturer, (normally qualcomm).
So the piece of hardware that can not handle dual lte is the soc (system on chip) which in mobile phones is the mind and heart of the phone, commonly containing graphic card, processor, storage controller and wireless connectivity (Bluetooth, lte, wifi).
With that in mind, one plus two have a snapdragon 810, this model of chipset does not support dual lte Sim, On the other side, one plus 5 have a snapdragon 835, which does support dual lte Sim.
Why is so hard to found if will it work? In part because there is a lot of confusion, from phone manufacturers and even within chipset manufacturers. Qualcomm documentation shows that both snapdragon 810 and 835 are lte dual active chipsets, but that is not true.
I can not stand for any other phone other than one plus 5 because that is the only one I have tried, but I presume that any dual Sim phone that have a snapdragon 835 inside should work with dual lte Sims (dual att Sims in the US for example).