In a mature liquid market, like exist for major US equities for example, you'd go long bitcoin and short bitcoin cash. Your broker would calculate the margin ratio on the total position and so you wouldn't get a margin call just because the bitcoin cash part went up, so long as bitcoin went up even more. There'd likely be some sort of carry cost.

But bitcoin, and especially bitcoin cash, doesn't have a mature liquid market, so this really isn't possible in any sort of off the shelf way. If you wanted to put at least a few hundred million into the position maybe you could get an investment bank interested. But even then they might not want to touch bitcoin cash.

If you can go short on either one, you can create any bet via a weighted + leveraged long/short pair bet. I think bitfinex allows this.

You can do it on Bitfinex by opening a long margin position on the BCH/BTC pair. Bitfinex requires a 10k account minimum and non-US residency though.

BitMex