You do not need two preamps. You can use Y connectors from a single set of outputs, that is what I did when I biamped. Use good quality Ys. It is not clear if you are using 2 matched pairs of amps or 4 different ones. In the first case you will need level controls for the pair of amps that is more sensitive, ie plays louder as a given preamp setting, so you can lower their output. Using mismached amps would require 3 level controls at least and be quite complicated to set up.
Details for bi-amping
I am going for a horizontal bi-amping of my system intending to use two tube monoblocs for the mids and the highs and two SS monoblocks for the lows. The issue is that I have to have the same gain of the Pre-amps driving all the four monos in order to avoid a difference. Some Pre-amps have a pair of outputs allowing them to drive two pairs of monoblocks. Would that be a good solution? Would that be a solution at all giving that the tube monos and the SS monos would most probably have different output power? Would the use of a single Pre-amp with two pairs of monos negatively affect the sound quality? Or I should go for two Pre-amps trying to match theeir gains? I will very much apreciate any hint. Tnx.