[AlaninCarlisle]

I always prune my cordon tomatoes to one main stem. However, for reasons I wont bore you with, I've allowed two main stems to grow on a few of them. As the plants are now well-developed and nearly a metre high, it seems quite drastic to take out nearly a third of the plant. Has anyone ever tried growing them with two main stems and does it affect the yield or health of the plant?

[Growster...]

I've had that happen on occasions, and tend to let them both grow, but perhaps limit the trusses to three or four per stem.

In fact, if you're feeding and watering them regularly, you've really got two plants for the price of one, so you'll get an excellent yield there!

[jambop]

I always prune my cordon tomatoes to one main stem. However, for reasons I wont bore you with, I've allowed two main stems to grow on a few of them. As the plants are now well-developed and nearly a metre high, it seems quite drastic to take out nearly a third of the plant. Has anyone ever tried growing them with two main stems and does it affect the yield or health of the plant?

I think a lot depends on what sort of growing conditions you are expecting. However to answer your question in a sort of way  IT is very common down here to have the main stem split into two and good crops are obtained... caveat , the weather is usually excellent for growing out door tomatoes and I use the old blue gravy to keep blight at bay 

[Aidy]

I allow a couple of side shoots to grow then I nip off and plant them in a pot, get the root system going and then plant them outside.
Just another thought for ya.

[mumofstig]

I often have tom plants with 2 stems, to make a double cordon - assuming the plant is strong enough to support the growth. No good trying it on a weak one.
I give the 2nd stem a cane and tie it in  as if it is a separate plant.

[Robster]

I deliberately did this last year in my green house and tied the second stem in to another cane.  It worked really well.  I paid extra attention to watering and feeding but I'm not sure this was so much a concern.  I definitely do it again no worries