Heal yourself without switching targets:
/cast [target=player] Heal

Heal a your target's target (if friendly), otherwise just heal whoever you're targeting.
/cast [target=targettarget,help,nodead] Heal; Heal

Voila. DPS class can keep dpsing while you heal. Using target=<something> is really helpful

You could also try something like this for your dps class - dps class will keep attacking the thing until it's dead, or you can have another macro to force targeting to a new mob. This is assuming your main is the leader of the party. Once their thing dies, they switch to whatever your party leader is targeting and continue dps.
/cast [harm,nodead] Fireball; [target=party1target,harm,nodead] Fireball