Well, not sure if there's another way, but this is pretty much it I think when you have a main pick and a saver.

How much you bet on the saver = how much you're willing to bet overall divided by the saver horse's price. Then what ever is left over out of the total amount is all put on the main pick.

So, for example.... 2 horses, main pick @ $3, saver @ $5, willing to spend $50, You would have $10 on the saver to get $50 and that leaves $40 to bet on your main pick to get back $120.

Personally, I would actually want to get a bit more back on the "saver". Seems a bit annoying to win and yet just get your money back.