We would open up ourselves to more litigation risk, by having too many options, the scenario of "it wasn't clear" is a much stronger case when you have multiple and varying options. With just two options it is clear cut.
If having options is a litigation risk then just have ONE option. Nobody will want to open two accounts just to have the ability to adjust their risk. People don't open a brokerage account for each security they own. Customers would find it inconvenient to pay bills from a balance that is split. If someone considers it simpler then consider you are solving what is likely to be a common issue by asking your customer to have two checkbooks, two ATM cards, two statements, two ledgers, etc.
Build your technology from the start so that an account has an allocation percentage. Make it as easy to adjust that percentage as is practical for you to implement. Consider that what your are providing is commodity-guaranteed banking and that every customer will have a different idea about which commodities are safer at the moment. You are providing a traditional bank as an alternative to risks from fractional reserve banking. Bitcoin shouldn't even need to be relevant; it is just one of the commodity choices.
The default should be an Euro-guaranteed account. You guarantee balances according to the commodity choices. You back that guarantee with bank choices. It is like how my bank holds gold reserves or bonds to back customer deposits. Customers need not see any more options than they'd be comfortable with, but the point of using your bank is that they have a choice to have their money fixed to something other than fiat.