What are your play times? I am going to go RDM/BST very soon and I'll be looking for a BST duo partner. This could help prevent you from having to level your WHM to 37 and you could sub WAR or even NIN.
FYI, a RDM sub for BST would work, but at the later levels your debuffs will not stick. If you were a 70BST/35RDM and fighting a Lv75 mob, then casting debuffs on it would be like a Lv35 RDM trying to debuff a Lv75 mob. The debuff might land but it won't last long. It works the same for mobs trying to debuff the player too. You know how when you farm and a casting mob will try to cast Poison one you and it doesn't stick very often and when it does, it lasts like 15 seconds? The same rules apply for us.
Dia 2 earlier than a WHM is nice, but Dia is about the only debuffs a BST cast on mobs at later levels because everything else is a waste of MP. The -na spells, Divine Seal, HP Regen and you get Clear Mind when your BST is Lv40 as opposed to 62 with a RDM sub. WHM is definately the sub to have if you're going to use a mage sub. It's also tough to beat getting the next level Cure spell earlier than a RDM.
Anyway, hope that helps some and let me know if you're interested. My RDM is 10 btw.
WoW: Climhazzard - Koral. Server - Night Elf - Lv70 BM Hunter
WoW: Shinerbock - Tich. Server - Orc - Lv80 SV Hunter
FFXI: Climhazzard - Hume - Siren - 146 levels of retired BST