Hi Mark,
I only use Sandisk compact Flash cards.....32GB 120MB's UDMA 7 Extreme to be precise.
Mine are used every day and on average I'm shooting between 300-1000 images per day.
You have to be careful with cards as there are a lot of fake versions about.
I have never (touch wood) had a card failure, Like you I have two slots and both cards are set to record the same files. If one did fail then at least I have a backup.
I have to do this shooting commercially to make sure my clients receive their images.
The same reason I have a second camera body and spare batteries......Bulit in redundancy I guess