The maximum penalty for a single count is 5 years imprisonment so, theoretically, a court could impose two five years sentences to run consecutively, giving a total of 10 years.
In practice, courts don't normally impose consecutive sentences so, unless there are extremely exceptional circumstances, a maximum term of 5 years is a more realistic answer to your question. (i.e. any sentences imposed are likely be to be 'concurrent' rather than 'consecutive')
The only exception is where the assault is racially motivated. The maximum penalty for a single offence is then raised to 7 years imprisonment.
Chris