Unfortunately the plugin only works with coin symbols being unique.
In your situation, I would recommend that you create different coin symbols, so that if your symbol is ABC, then create something like ABC and ABC2, ABC and ABCOLD, or ABC and ABC2, or something like that.
If you absolutely need to use the same symbol for both coins, then how are your users going to differentiate between the two? Only by the coin’s icon? You need to explain to your users why they have two different balances on what appears to be the same coin but is actually two coins.
In any case, here’s an ugly hack that I thought of just now: You could maybe use a non-printable Unicode character, such as the zero-width space, in the old coin’s symbol string: https://en.wikipedia.org/wiki/Zero-width_space
I am not saying that this is a nice solution; it will probably create confusion, so it’s up to you to decide if you want to do this. If you do, at least make sure that the two coins have different icons, because this will be very confusing for your users otherwise.
The two coins will be different for the system, but users will be seeing the same symbol on their screens.
I hope that this helps you, and that it doesn’t create a big mess! Please test it before deploying on production.