How many ISA's can I have at once?

The rules for stocks and shares ISAs are the same as with cash ISA’s.  You can only pay into one each tax year, but can open a new ISA with a different platform each year if you wish to.  If you have multiple stocks and shares ISAs open, you are only allowed to pay into one of them in each tax year. 

If you only wanted to invest a portion of your ISA allowance via your second ISA provider, this could be problematic as it will mean you are not able to add any new money to the original ISA in the same tax year.  The other option to consider is transferring your existing portfolio to the other provider, although the cost needs to be carefully considered.