Using an account different from the one used the previous year (no need to have more than two) can make the net-income calculation a bit easier if you end up wanting to recharacterize later, but is definitely not required. The calculation can still be performed when only one Roth IRA is used, it just takes a bit more work. This is true for any Roth conversion, for backdoor purposes or not; "backdoor" is simply slang for a series transactions, one of which is a Roth conversion like any other.
