If you have more than one participant in your retirement plan, each participant’s funds must be accounted for separately. This can be accomplished with either the use of spreadsheets or accounting software without the need for a separate bank account. This can also be accomplished with separate bank accounts if you prefer. Do what works best for you and your CPA to keep the books clean!
The same can be said for the accounting of Roth and non-Roth funds if you have both within your Solo 401k. However, since these funds are taxed differently, many choose to have separate bank accounts to hold Roth and non-Roth funds for clearer separation and more simplified accounting.