If you are arbitraging between 2 spreadbet firms then you need to fund the margin requirement for the trade on both accounts, therefore doubling your overall margin and halving the overall return for the money that you have tied up in the accounts.
Also you need to have enough in the account not just to cover one trade but also a possible series of losing trades. If you spot an arb opportunity on the dow for example and then can't find a reversing arbitrage trade then you need to run it until the end of the day by which time one of your accounts could be showing a big loss, albeit offset by a big gain in the other account. You then won't be able to get the cash out of one account and into the other to make up the loss before the next day's trading.
All in all, you'd be better off just putting the cash in the bank!