suppose you take a signal, and duplicate it, then apply a low pass and high pass filter both with the same frequency and roll-off respectively to each signal, then combine the two signals additively, will that be a good approximate for the original signal? (guessing from some experimentation in audacity, this seems to be true of white noise at least)
As long as you have identical rolloff characteristics - slope and frequency, the mix of the two will be identical to the original signal.
Whether the filter alters phase or not depends entirely on the design of the filter. For instance a basic first order R/C filter will alter the phase by between 45 and 90 degrees depending on slope and frequency. There are many different filter designs which can be emulated by IIR/FIR filters in the digital domain.
As far as frequency and level go, you're good to go.