I had to register here just so I could answer your question: YES! Do it!
I play as a Holy Pally (with Retri as off-spec) and my brother plays Prot Pally, and oh my goodness - we are a formidable force. And one to be reckoned with at that!
We didn't level up together, however at level 80 we do everything together. Dailies / group quests that require more than one person (I.E. Chillmaw from 'Threat from Above') can be solo'd by my brother. However, it's not fast. So I come along and speed things up with my DPS, or ensure survivability as Holy.
We do heroics together. We have tank and healer down already, rounding up DPS is no problem. This cuts down LFG time significantly. I also don't have to worry about the dungeon itself taking very long - my brother is an exceptional tank, and he trusts me as a healer completely.
We raid together. If being tank / healer combo wasn't awesome enough, we supply amazing raid buffs. Usually 2 pallies is more than enough for a raid, so (even though it sounds a bit selfish) your wife shouldn't have to worry about rolling against other healers for plate gear. I've geared up pretty fast from this happening.
And then there's Dual-Spec. This completely transformed my pally experience - after 2 years of healing, it's safe to say that I was getting pretty darn annoyed with having to do my dalies as Holy spec, and didn't have the time to level up another alt. I love playing pally to bits, so having an instant level 80 Retri pally was just amazing. It felt almost like a new character, and opened up so many doors for me. My brother benefitted from me having Dual-Spec also - we now farm BC heroics together for rep / mounts (MgT for the White Hawkstrider) and blitz through them.
Two pallies really is, in my eyes, the perfect duo. We can take a severe beating due to being plate wearers, bubble out of tough situautions, Lay on Hands ourselves / each other... the list is endless. I highly, highly recommend the class for levelling as well and end-game purposes.
(Also, I see alot of people say that Holy Paladin is 'boring'. I completely disagree. I love being able to do the biggest heals, I love the fact that I can do massive instant heals with Holy Shock and I love having the biggest mana pool due to Intellect being an awesome stat for Holy Pallies. I like the way my Paladin casts and the fact that it's my favourite race (Blood Elf), making me enjoy playing Paladin even more. Because of this, I'll probably never end up playing an alt seriously. Paladin crosses all the T's and dots all the I's for me.)