Actually this is exactly what major investment managers do with cash. There are services provided by major banks (e.g. State Street) to the IM industry that do automated sweeps into multiple bank accounts (hundreds or thousands of accounts in some cases), repos etc. Keep in mind that typically mutual funds, CITs, etc. are structured as separate corporate entities with separate cash books, so even though (name your major fund company e.g. Vanguard, Fidelity etc) may have trillions in assets, each individual fund may have a much smaller amount under its name.
While I can't speak about Tether, hypothetically speaking if you had a single legal entity with $2B in cash, I would expect it to be well diversified in very safe and managed pools, either bank accounts (up to FDIC limit), repos, CDs, or other safe short-term instruments.
Source: investment management industry knowledge and personal experience with the same.
