if I revealed an Undertitan with Rek'sai's Legend ability, can I use the 2 energy added by the triggered effect of the Undertitan right away to pay the cost for the card that is to be played as a result of the Legend ability?

Yes — the 2 energy is in your Rune Pool before you pay for the played card.

Undertitan's "[Add] [2]" is a replacement effect on the event of it being revealed (Rule 369.2/370.1.b.1), so the energy lands as the top 2 are revealed — which happens before you banish one and play it. When you then play the chosen card, costs are paid during finalization (Rule 357.1), and you can spend that floating energy there, the same way FAQ #1271 confirms you can tap/recycle runes to pay costs during this ability's resolution.

Two caveats: this only works because Void Burrower says "reveal" (look effects give nothing — FAQ #1708), and unspent energy is lost at the start of your next Main Phase (Rule 167), which won't matter mid-resolution.