Hi,
You can pay into two ISAs in the same tax year provided they are different types of ISA.
It would be fine to pay into both a cash ISA and a Stocks & Shares ISA in one tax year as long as you’re below the £20,000 limit.
You would not be able to pay into two different ISAs of the same type. You should contact the second provider and advise them of the situation.
They will most likely, refund the payments into the ISA and close or void the ISA. More guidance can be found here
Individual Savings Accounts (ISAs)
Thank you.