No. Valakut's trigger specifically requires 5 other mountains. So even though the two mountains entering play will essentially "see" each other, Valakut's burning does not start until the sixth mountain enters play.
They come in at the same time and see each other but I think you have to have 4 in play before casting the titan to get the damage. The key being 5 "other" mountains. So short answer no. If he had had 4 when he cast it then yes.