thanks，more questions

5.if I use same SSID for two routers both have 2.4G, I think they will mess up? because both will handle same packet right?
6.if one router uses different ssid(by default) for 2G/5G, then whole bandwidth (2G+5G) cannot be used on single device right?
7.the whole bandwidth is doubtful even with multiple devices, for example printer is on 2G and PC on 5G of different ssid, can they comm?
8.the last cfg is to use same ssid for 2g/5g on same router, yet I'm not sure the router support it or not. have you ever tried?
9.I won't switch to one wireless router before I know the new one is robust.