What the rebate form actually says is:
"If applying for more than one rebate, all submissions must be enclosed in same envelope. Multiple submissions mailed separately will be denied."
I took this to mean that I send 2 rebate forms in same envelope, and that is what I did when I sent off for 2 previous rebates on Kingston SD (256 MB) cards.
I don't know if Kingston is picky about which way you submit, but they DID accept 2 separate rebate forms in the same envelope on a previous rebate offer. I DID get the rebate checks from that rebate offer about a week ago.