Sleeplessmom, I think you and your husband need to sit down and discuss the financial side of it first. Can you even afford to send one? Can you afford to send both? What does this mean for the rest of the family?
When you figure out if you can even afford to send one or both (lets assume you can for a moment), then sit down with your kids. Have them discuss why they want to go to Israel or do they just want to further their education?
I personally hate when people say “I am going to Israel to grow.” Does growth not happen in America? Doesn’t it start right now if you actually want to change? I am assuming that you are a nice frum family and that there arent really dangers that your kids need to leave your house to grow in Torah and Mitzvos.
I like Gavra’s suggestion of having your kids pays for the experience – at least discuss it with them. They may not want to go once they realize that its their $20,000 they are spending. If you can only afford to send one, perhaps offer to pay for half of each and they will come up with the other half?
This is a luxury, albeit a really wonderful one. Good luck with the tough decision.