Is it legal for a 501c3 to have a fund raiser solely for the purpose of making a donation to a different 501c3?

Yes - this happens all the time. However, I would recommend that you do this only if the purpose is reasonably consistent with your mission, or you will potentially confuse or alienate your constituents, and with board approval. For example: a church raising funds on behalf of the Red Cross for relief in Haiti makes sense. But that same church raising money on behalf of Planned Parenthood may not. In any event - be sure that you are open about who the money is going to, and why it's appropriate for your 501c(3) to be doing this.