For simplicity, let's say the bankroll consists of 100 BTC, 550 BTC after margin from 2 investors.
One investor has 50 BTC at 1x margin, the other at 10x margin. Bets would be taken as if the bankroll was 50x1 + 50x10 = 550 BTC. The effective bankroll of 550 BTC is not an issue, since the max that can be won in any situation is around 1-5% of the margin bankroll depending on the game, meaning even if every investor was at the maximum margin, the largest win possible would not be larger than the bankroll, limited to about 10-25% of the real coins backing the bankroll.
If a player won 10 BTC, the investor at 1x margin would pay 50/550 * 10 BTC = 0.909 BTC to the player. The investor at 10x margin would pay 500/550 * 10 BTC = 9.09 BTC.
If a player lost 10 BTC, the investor at 1x margin would earn 50/550 * 10 BTC = 0.909 BTC from the player. The investor at 10x margin would earn 500/550 * 10 BTC = 9.09 BTC.