.... Smartphone .... about 6-7".
That's not a 'phone, it's a small tablet.
I currently carry two or three mobiles at any given time, and one of those is a dual SIM. I can't help you with the size (6-7" is way too big for what I'd want, so I've never looked for something that size), but some general considerations.....
AFAIK, there still isn't a dual SIM 'phone that doesn't relegate the second SIM to 2G-only. Even with the relatively few 4G dual SIM 'phones now available, only one SIM gets to do (up to) 4G; the other is stuck on 2G. As carriers start ditching 2G (and some overseas already have, or never had 2G in the first place), this is going to become more of a problem. Even in many places in Australia where there's good 3G and 4G coverage, 2G can be patchy or non-existent, so it's already a problem here for some.
Most dual SIM 'phones are "dual standby"; when you're talking on one SIM, the other is effectively off the air. If you want the other SIM to remain on air whilst you're on a call you need "dual active".
Make sure you get one with all the bands you need, or think you might need. Availability is much better these days, but when I bought my first two dual SIM 'phones (I'm currently up to my third), multiple bands weren't common or even available.