i was looking for this as usually create a different names ssid like wifi and wifi-5g so on but changeing the name i find i can't manually switch between them but device is actually doing it by own..
That's why we have 2.4G+5G on same SSID and then "band steering" to allow client/AP to switch over.
Though this might not always working as expected, for my Asus RT82U router (non-OpenWrt compatible) I can set various threshold so that AP can do cut off more aggressively.
I've never liked it as a feature. I seem to end up connected to slow 2.4G networks for no apparent reason.
2.4GHz has slightly higher range than 5 GHz. There is a point at the far distance, if you sit in the middle between the outer limits of these ranges and using same SSID, your client device may oscillate between the two bands, if you just move slightly. Lot's of lagg and many disconnects. I experienced this hell with OEM software, which was not configurable. Since I installed OpenWrt and use different SSIDs, I have no problems anymore at this exact location 
There is also the phenomenon of client devices "sticking" to one band and it might not always be the most performant one.
I hear people being able to achieve great client roaming performance with 802.11r as well as 802.11k or similar protocols, having fine tuned their networks, but I have not tried it (yet), so no comment on that.
At the end of the day, it's a policy decision - both is possible, both can be valid choices.
For most users, sharing the same access credentials between 2.4- and 5 GHz makes sense, especially as these days most client devices should make sensible roaming decisions. If the clients are less smart, there is a risk of oscillating or sticky clients, but at the same time it helps you to retain your connection if you actually go beyond the range of your 5 GHz radio.
Using separate SSIDs can very well make sense, especially if all your interactive clients are 5 GHz capable (leaving 2.4 GHz only to IoT- and similar low-speed and stupid 'smart' devices) or if some of your interactive clients expose the behaviour cited above (and if you avoid the complexity of active band-steering).
As mentioned, depending on your priorities and -devices, either of these can make sense - and either is a totally valid policy decision, just give it a try and test it for yourself.
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