Hi edgar,
If fed L and R respectively from a stereo source, yes they will produce stereo. Theoretically if both channels are matched perfectly they will have an advantage in imaging over a stereo preamp whose two channels share a common power supply. It's the same principle as monoblock vs. stereo amps. There is one huge caveat however and that is, as mentioned, having both channels as closely matched as possible. By that they will have to have pretty much matching tolerances for all parts end to end. Mismatched gain can skew the soundstage and so can distortion. Timing differences will likely be perceived as mismatched gain, as in later will come across as softer with the image pulled toward the earlier or stronger channel. If the two mono preamps don't have stepped attenuators it will also be difficult to set the channel balance with the two independent volume controls. On the other hand two independent volume controls can serve as a substitute for a balance control. Personally I'm used to dual mono preamps and in time anybody that uses them will get used to them too. Just remember the caveat.
HTH
Jack