VAT
If you are liable to VAT, you are to submit regular VAT declarations and pay VAT.
A number of activities or independent professions (lawyers, notaries, bailiffs, etc.) are granted a VAT-exemption. If you are granted a VAT-exemption, you will the lose the right to deduct VAT.

Taxes
The income of both activities will be combined. That could result in higher tax rates if certain income amounts are exceededed.
According to your income, you must make three-monthly advance payments. It is strongly recommended to make these advance payments. That way, you can avoid a tax increase.
If you establish yourself as a self-employed person for the first time and you are younger than 35 and you mention this in your declaration, such increases will no longer be charged.