If you wanted to go real high end and give it a real faustian bargain vibe, could do two castings of Demonic Blood Infusion and Extract Gift on the promised lovers.

Demonic Blood infusion lets one count as a Demon temporarily, whilst Extract Gift allows you to grant a recipient certain bonuses with the option of also giving the Demon donor certain powers over the recipient, namely:

+1 Increase: The donor demon can track the recipient's location and condition, effectively gaining the effect of a constant status spell on it.
+3 Increase: The demon can see through the recipient's eyes and hear through its ears.
+5 Increase: The demon can telepathically communicate with the recipient as if by Rary's telepathic bond.
+7 Increase: The demon can use demand on the recipient once per day as a spell-like ability (caster level equals your caster level at the time you cast extract gift).

I believe it works across planes as well, unlike the base ones, although that might technically be debateable.