As @IgnacioVazquez-Abrams noted, the difference in speed is potentially a real problem. Data transfer requirements can take a variety of forms. For your purposes, let's look at a few possibilities:
1 - A bunch of small, intermittent, packets in both directions
In this case, any small device with 2 UARTs and enough RAM to buffer the largest typical transfer will be sufficient.
2 - Large amounts of data coming from the slower device with acknowledgements or other small packets being returned by the faster device
In this case as well, any small device with 2 UARTs and enough RAM to buffer the largest typical transfer from the faster device will be sufficient.
3 - Large amounts of data coming from the faster device with acknowledgements of other small packets being returned by the slower deive
In this case, I recommend researching to find a different solution. While it is possible to create ever-larger RAM buffers for serial data, eventually, inevitably, there will be a problem.
Note that I use the term "packets" here very loosely. The data does not need to conform to anything like Ethernet network packets or similar highly-structured data. But most data transfers, in my experience, beyond keyboard input, fit one of these 3 models.
There is an additional factor - handshaking. Either device, or both, may be able to handle incoming data at the full defined speed. But one or both may require handshaking to be implemented in order to function reliably. That handshaking may be hardware (CTS, RTS or other pins) or software (e.g., XON/XOFF). If you can't configure the bit rates then you are unlikely to be able to configure the handshaking and should support whatever each device needs. Again, a small device with 2 UARTs can handle different handshaking methods as well, but needs to be configured properly to do so.
I am also a little surprised that you would have 2 devices that BOTH have fixed bit rates. Most devices I have worked with for the past 30 years have had configurable bit rates. Even if one device is fixed due to other constraints (e.g., you are capturing data that is already being sent to another system), that typically doesn't apply to BOTH parts of a project.