thiago.ndias wrote:
I'm installing an app that requires the file it's going to read to be on the same disk where the app is installed,
That sounds like a very poorly designed app. Poorly designed apps generally end up causing even more problems. I would suggest looking for an alternative app. Plus with all the new macOS security features, that type of setup may not even work after a macOS update. Be careful with such an odd custom setup since macOS does not do well with odd customizations.
I agree with @den.thed, that using a single larger SSD is the better option since the price of two smaller SSDs will generally be about the price of a larger single SSD. Combining multiple drives involves using RAID which can be problematic for users unfamiliar with dealing with RAID arrays.
Simpler setups are much easier to maintain. The more complex the setup, the more likely something will break.
Also, you will need to backup anything stored on that external drive if that data is important. People should always have frequent and regular backups of their computer and all external media (including the cloud) which contains important & unique data.