Yes, you can store your funds in both SGD and USD under your buying power. However, the total 'Cash' amount reflected in your app Home page is displayed in your chosen reporting currency.
For example, if your chosen reporting currency is USD, the displayed 'Cash' amount in your app Home page is calculated as the sum of USD held in your buying power plus the USD-equivalent of the SGD held in your buying power (based on most recent foreign exchange rates).
To see the breakdown of SGD and USD funds held in your buying power, simply log in to your Syfe Brokerage app > click on Accounts (three lines at the top right corner) > choose My funds > click on the '?' icon next to the word 'Cash'.
Learn more about your buying power: