Can spells with cumulative or instantaneous ongoing effects affect a creature multiple times?

In the current campaign, the levels 2 and 3 PCs want to spam foes with the 1st-level Sor/Wiz spell cause fear [necro] (PH 208) and the 1st-level Sor/Wiz spell power word pain (Races of the Dragon 116), the former so that they can corner and beat into unconsciousness the rare creature that they want to take alive and the latter so that they can perpetrate dungeonwide loot-their-corpses-after-they-die-in-4d4-rounds murder sprees.

It seems reasonable that a creature affected twice by the spell cause fear becomes frightened rather than merely staying shaken for a new, longer duration. Likewise, it seems reasonable that a creature affected by two or more power word pain spells would be in more pain—and suffer more damage—rather than the new power word pain spell’s duration overwriting the previous’s.

Nonetheless, since these aren’t, like, polymorph effects or temporary hp or anything with a specific rule, I am consumed with self-doubt and suspicious of both tactics, thinking they conflict somehow with the rules about the Same Effect More than Once in Different Strengths (cf. Player’s Handbook 172), the rules about the Same Effect with Differing Results (ibid.), or both… or another rule somewhere.

Can these kinds of spells—that have ongoing durations but either cumulative different effects or instantaneous effects—affect the same creature more than once simultaneously?