Newbie - too late to prune?

[Nancy1120]

Hi,

This is my first year growing tomatoes. I'm growing out of city pickers (earth box wannabe from homedepot). I planted two tomatoe plants per box (wish I had just planted one). My question is - the boxes are getting a little too crowded. Is it too late for me to prune my plant down to one stem? The second stem is as thick as the first stem and it's starting to flower. They've been in box for about 1.5 months.

Thanks!

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