You do not need two preamps to run two subs. Here are cases with one preamp and two subs:
(1) Some HT processors have two sub outputs, in anticipation of users with two subs.
(2) Some 2 channel preamps have redundant full signal outputs. They can sometimes be used to run two subs, though I would check with the preamp manufacturer before choosing this setup.
(3) Some subs (e.g., JL Audio) are capable of a master/slave configuration, so that the output of one sub (the slave) is controlled by another (the master). In this case, the subs are mono.