Until you are married then you will be taxed as individuals, the authorities here assuming you are both foreignors will be a duly authorised copy of the marriage certificate.
However, do not worry about it, it does not make a huge amount of difference unless one of you is on a relatively small salary and one on a very large salary, if you both earn roughly the same then forget about it.
Taxes are charged on taxable income and start at 6% rising to 40%, but you will need taxable income of NT$ 4 million or more to qualify for the upper bracket.
Tax allowances come in various forms and some are standard and non changeable, but some are standard but changeable, and the decision as to which to use has to be based on personal circumstances.
If you want more details then just ask away.