All the fast trains have seat reservations, so a ticket is valid only on the booked train. You will need 2 tickets.
The price depends on how far ahead you book. Trenitalia tickets go on sale 4 months before travel, and the cheapest tickets often sell out quickly. Discounted ticket prices keep going up as the travel date gets closer.
You can get train times and costs from www.trenitalia.com clicking on the UK flag to change to English and using the Italian names for towns Roma Termini, Firenze SM novella, Venezia S Lucia), or from the competitor on major routes http://www.italotreno.it/EN . There are notes about online booking at www.nickbooth.id.au/Tips/ItalyTrain.htm
@jamek247 As Nick has indicated, buy 4 months before travel for the best prices. Typically, if you buy that far ahead, you will be able to get the cheapest discounted fare of about € 19 per person for Rome to Florence.
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