Yes, you can open multiple Fixed Term Savings accounts if you wish to diversify your terms and interest rates. However, be mindful of the FSCS protection limit of £120,000 per bank. If your total savings exceed this amount, any additional funds will not be covered by FSCS.
Please be aware that if you already have for example a 6 month fixed term product with GB bank, you will not be able to open another 6 month fixed term product with GB Bank at the same AER.