Are you interested in feeding the left channel to one sub and the right channel to the other? Or summed left and right to both subs?
In the first case, any preamp with two sets of outputs would work. I know of several with dual RCA outputs: Kora Eclipse, Anthem Pre 2L SE, Aloia 11.01. And a couple with one XLR and one RCA output: Threshold Stasis-R 3.0 or any BAT VK 30 or 31. Many others also will have dual outputs.
If you want summed left and right to both subs, you could accomplish that with "Y" cables and preamps with dual outputs (one output for the main amp/speakers and the other for the "Y" cables to the subs) or find a preamp with 3 sets of outputs, such as suggested by "Shion ca".
What kind of inputs does your amp have? What kind of inputs do your subs have?
Disclaimer: I have some of the preamps listed above for sale now or may in the future.