You can as long as both ISAs are with NatWest. We'll allow you to save into both a Help to Buy: ISA and a 'regular' cash ISA in the same tax year as long as your overall ISA savings (including any other stocks and shares ISA fund you may have) don't exceed your annual ISA allowance. In the current tax year, this is £20,000.
As an example, you could save £2,000 in your NatWest Help to Buy: ISA and up to £18,000 in a 'regular' NatWest cash ISA.