separating the SSIDs in two bands will enable you to be sure the devices in one SSID always connect to 2.4 Ghz and to 5 GHz in the other SSID, but in terms of throughput It is similar as having 1 SSID for the two bands. In Unleash architecture there is not much more you can do to increase the throughput as you can in solo/ZD/SZ architecture, as disabling low speed modes for old devices.
If the mobile phone is 2x2 It will use 2 data streams.
The spatial-streams on the R600 apply to both the 2.4GHz and 5GHz radios. Therefore you can theoretically connect a 2x2 client to the 2.4GHz radio and a 3x3 client to the 5GHz radio simultaneously.
The 5GHz radio on the R600 is 802.11ac Wave1 capable, whereas the 2.4GHz radio is 802.11n capable, so there are higher modulation data rates available to 5GHz clients, as well as wider channelisation (upto 80MHz), which is not available on 2.4GHz radio.
However bear in mind that the bottle-neck in any enterprise-grade network is usually the client device. You can configure your AP for the highest possible performance but if you connect a 2.4GHz 802.11b 1x1 SISO client on a 20MHz channel, it will only be able to achieve an 11Mbps data rate. This still applies even to the latest Wi-Fi 6 mobile client devices, which will normally have performance restrictions due to spatial-stream, NIC, CPU and RAM limitations, compared to the capability of the network infrastructure. Also the network infrastructure is designed to be higher-performing than a single client as it is intended to connect large quantities of client devices simultaneously.
You're most welcome. The Wi-Fi and TCP-throughput performance capability of the iPhone 11 is a question for Apple, as even though it is a Wi-Fi6 2x2 MIMO capable device it may not support all MCS data rates and 80MHz wide channels as well as inherent hardware limitations.