If you want to connect a device (keyboard or whatever) that has a USB-MIDI port to a device that has standard MIDI ports, or connect two devices that have USB-MIDI ports to each other, you'll need to connect them via a third device that can function as a USB host.
If you're at home, this third device would typically be a computer or laptop, although a tablet or smart phone could also work as long as there are no issues (see below).
If you're in a situation where you can't-- or just don't want to-- use a computer, laptop, tablet, or smart phone as the third device, there are certain types of USB-MIDI interfaces that can function as a USB host device (see below).
The simplest method is to connect both keyboards to a computer via USB, then use software to route MIDI data back and forth between them-- e.g., you can set up one or more MIDI tracks in a DAW with one keyboard as the MIDI Input device and the other keyboard as the MIDI Output device; or you can use a simple utility (e.g., MIDI-OX) to route MIDI data from one keyboard to the other.
If a computer or laptop isn't a desirable solution for some reason, you might be able to use a tablet or smart phone as the connecting device, although there are a few issues that you might run into.
One issue is that most tablets or smart phones either have some sort of special port (e.g., for a Lightning connection), in which case you'll need a special adapter that lets you plug in a USB cable; or they have a mini or micro USB port, in which case you'll need a special USB cable or adapter to go from a standard-size USB plug to a mini or micro USB plug.
Another issue is that most tablets or smart phones have only one port for connecting a USB device, in which case you'd normally need to connect multiple USB devices via a USB hub-- except (if I'm not mistaken) Yamaha says that you shouldn't connect the PSR-I455 to a USB hub, because it requires a direct connection to the USB host device.
One way you could get around those two issues is by using Bluetooth MIDI connections, assuming the tablet or smart phone can use Bluetooth MIDI. The PSR-I455 doesn't have built-in Bluetooth MIDI capability, but Yamaha makes the UD-BT01 adapter and it should work with the PSR-I455. You'll either need to get two UD-BT01 adapters-- one for each PSR-I455-- or you could use a single UD-BT01 adapter to connect one PSR-I455 to the tablet or smart phone via Bluetooth MIDI, and connect the other PSR-I455 to the tablet or smart phone via cable.
Another possible issue with using a tablet or smart phone is that it might not have a built-in ability to function as a USB host device-- e.g., some Android devices might lack that ability, especially if they're running an older Android OS. There might be apps or drivers which can be installed to add USB hosting ability to the tablet or smart phone.
The other approach would be to get a USB-MIDI interface that can function as a USB host device.
The Kenton MIDI USB Host is one such interface; it can connect a USB-MIDI device to a standard MIDI device. If you want to connect two PSR-I455s this way, you'll need two Kenton interfaces-- one for each keyboard-- and use MIDI cables to connect the two Kenton interfaces to each other.
Or you could get an iConnectivity interface, as long as it's one that can act as a USB host device-- e.g., I don't think the mio2 will work (it doesn't have a USB "B" port), but I think the mio4 might. On the other hand, the mio4 has only one USB "B" port, so the only way to connect two or more USB-MIDI devices to it at the same time is to use a powered USB hub, and I'm not sure whether that would be a problem for the PSR-I455.