whoever has large bankroll takes the other one into it 
how much big bankroll needed to play at dice but we have to keep in mind that thing losing streak with 2x payout can go up to 30 in a row if got that kind of losing streak with martingale than we can overcome or not? using martingale is always risky where house edge exist.
its simple as that ... explained below
you have 10 BTC and site has 1 btc then start with 0.05 btc bet and most probably u will eat the site's BANKROLL