So you'll want to use the split contract to send the amount you have in both ETH and ETC. You can check your ETC balance here: http://gastracker.io/addr/0x..... If you try sending more ETC than you have, the ETC transaction will fail.

So let's say you have 100 ETH and 10 ETC. You should call the Split function using 10 ETH (minus gas costs). Now that address will have 90 ETH and 0 ETC. You can then send the remaining 90 ETH normally, without needing to worry about replays.