When a person pays....how do you split a payment between two accounts?
Using paypal and stormpay?

The answer is...you don't. You can not accept a payment to both at the same time, you need to accept to one or the other. Being that these are both 3rd party processors, they will require the user to visit their website at some point during the transaction to complete the payment.

it is better to keep in one account for admin purpose.

Paypal does have that IPN or something like that. You could possibly retrieve the value that was paid & then send that over to Stormpay. of course - this seems like more work than really needed.