I've read that most dual-SIM phones can only use the other SIM for 2G voice service.
No, OnePlus 2 is not any different.
In fact, users on the forum report that the dual SIM functionality apparently requires the presence of a 2G GSM network.
I have the OnePlus 2 (A2005) and am using it in Canada. SIM 1 on Telus (4G/LTE) and SIM 2 on Wind. When I want to use data on Wind, it switches SIM1 down to 2G. Except neither Telus nor Wind has 2G. So when I choose either one for data, it shuts down the other one completely due to lack of 2G. It doesn't just step it down to 3G. This makes the phone pointless for me. Due to their advertising I was hoping that both would be 4G simultaneously, not 4G on one and 2G on the other.
You can connect to any network from any SIM (e.g., can make either SIM a primary one)
The non-primary SIM can only connect to a 2G GSM network
If your non-primary operator lacks a 2G GSM network (which are being shutdown worldwide, and some GSMA operators never had any to start with), the non-primary SIM cannot connect to UMTS or LTE, and will thus be getting no signal at all
Off topic but not by much ... I have a Galaxy S7 Duos. My SIM 1 is T-Mobile set to "Auto" band (LTE/W-CDMA/GSM), and my SIM 2 is a foreign SIM that roams on the AT&T network here in the US - set to (GSM Only).
As OP said, their phone (and most dual-SIM phones that I'm aware of) only allow 3G/4G to run on ONE SIM, and the other SIM has to use GSM.
That's how it was on my S7 Duos for most of 2016. Then after updating the firmware recently (Marshmallow CPK5 if anyone's interested), My SIM 2 was now magically capable of doing "Auto" band (3G/GSM), and my SIM 1 was STILL capable of doing "Auto" band (LTE/W-CDMA/GSM)! Of course, from what I can tell, SIM 2 is just using UMTS bare-bones 3G - nothing fancy. But it's good enough.
I think phone manufacturers are wise to AT&T shutting down their GSM network by 2017. My update came in November 2016. So maybe other phones will get similar updates before 2017? Check around, contact your manufacturers. Good luck!