Depending on the content, I usually watch the other WHM and start doing whatever they're not doing. If I see a Cure going out, I'll toss a support spell, and vice-versa. I don't let anyone -na over me unless it's a huge AoE that hit more than two or three people, though, in which case I watch and see who they're casting the -na on so that we don't overlap and waste each other's time.
This!
If there are other healers, I try to work together with them rather than compete with them. I'll sometimes even talk to them beforehand to split duties if I trust them.
I really hate when WHMs try to compete to cure the same person as me, because it's very counter-productive and not just because it wastes MP. It wastes a party slot, too. What I'll often do if I'm not in the party that's taking the most damage, is act as a backup healer for the WHM that's in that party. I'll watch who they heal, and say one person takes a bunch of damage and the WHM in that party starts casting Cure V. Then a second person in the same party takes damage and is in the red, that's where I step in to Cure the person whose main WHM is casting on someone else.
If there's enough actual healing to warrant multiple WHMs they should work together, not compete. Nothing bothers me more than having one person in my party die while I'm healing another, especially if a WHM in a different party could've saved them but instead wanted to try to beat me to my own Cures. They should know they have no chance of beating me! >:)