Is it possible to split the 2.4ghz and 5ghz networks on the Orbi

It's important to know which Amazon device you are using as well as which content you're expecting to stream. Finally, is there a problem to begin with? Let me address the obvious one first. Do you have a problem with buffering at all? Have you noticed a "spinning wheel of death" ,or a loading pinwheel, at video start times? If there is no problem, strengthening the signal will offer no realized gains for you at all. As far as the device and content expected goes; if you have a 4K capable device plugged into an HDR television, that content (proper high definition, not YouTube's "4K" resolution videos) may demand upwards of 35Mbps. If we use the speed that most people are familiar with, Megabytes per second (MB/s) rather than Megabits per second (Mbp/s), that's just under 5 Megabytes per second. Not only is that well within 2.4Ghz frequency capability, but a mechanical USB 2.0 hard drive can keep up with that speed over the network as well.

There's no  reason at all you would need to prefer one frequency band over the other unless you are reaching the maximum range of the much shorter, but stronger, 5Ghz band (there is also the possibility that if you were on the 2.4Ghz band and were encountering an incredible amount of interference, in which case you'd require a stronger radio broadcast in order to take advantage of the 5Ghz band). In that case the signal strength would drop below a set threshold configured in your device and adapt itself to the appropriate band to maintain the signal strength above that threshold. In network devices that do broadcast separate SSIDs, that would not be the case, and you would have to select one or the other. If you find that you are experiencing issues, do as another has mentioned and try and minimize the distance between the Fire stick and the network, and do what you can to mitigate the amount of solid obstacles between their line of sight. If you still have trouble and you can't move the devices any closer, you'll need an additional access point to expand the coverage of your network unfortunately.