With a single druid, you can manage the procs.
First you need a way to notice them.
And then you need an easy way to switch between the two.

Or alternatively, don't even switch but have your fall through something like:
/castsequence Wrath, Wrath, Wrath, Wrath, Starfire, Starfire, Starfire, Starfire.
Which while not managing the proc, will improve your DPS a lot over straight Wrath or straight Starfire spam.

The Boomkin, even if its own DPS sucks, significantly ups the DPS of every other caster on the team.
So the choice is hurting everyone's DPS somewhat and replacing the boomkin.
Or continuing with the Boomkin, and either actively managing the proc, or approximating the proc.

If you choose to replace the Boomkin, I'd pull him right off the team, instead of switching roles with the Priest.
Look at adding a warlock to the team, in the boomkins place, if that is at all feasable for you.
If you don't have a warlock now, that is a lot of leveling, which can really suck.
So maybe Resto Druid and Shadow Priest is the best bet, but I'd prefer Priest Healer + Boomkin or Warlock.