I have a dual sim phone, a Vivo Y28.

It only works in dual sim mode with one sim card selected for 3G. There is no way to select 3G for both sims, and if 3G is turned off only one sim card works.

I currently have an AIS sim and an imobile3GX sim (which is an mvno working on the TOT 3G network without AIS 2G back up, so only works in built up areas).

As standard, I run my AIS sim on 2g and the imobile3GX sim on 3G. My AIS sim is for incoming calls, though I do top up occasionally to keep it from expiring and for when I am out of the city. The imobile3GX sim is always on data (25satang/MB) and I use for calls (49satang/minute). Its really cheap but unreliable. I can't use it without the AIS sim as back up.

With the end of AIS 2G in a couple of weeks, am I going to need to select 3G for the AIS sim for the dual sim functionality to continue? It seems only 3G phones will work, so a load of old guys are going to find their phones no longer working. And does anyone know a decent package for unlimited 384kbps monthly on AIS? I don't need 3G on my phone, enough to send a receive emails.

Dual sim phones usually support data use of 1 sim card only, the 2nd sim is phone calls only. At least that is what some Samsung and Sony phones were like the last time I looked at.

Nope, the 3G sim card and phone itself won't become useless, but it in one card slot dual sim phone supports data and calls, the other slot only calls.

Currently I can make calls and use data on both sims. Obviously only one at a time. Before each call I have to select the sim to call out with. My wife's Oppo is the same.

I would not worry until/if they actually end 2G service - there are still a number of people using this service so suspect there will be normal delay in ending - even if they have to rent frequency space from another provider.