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You didn't say whether these are: (a) two completely separate Comsol models, or (b) one model, but with two independent but sequential sets of physics/computations, or (c) one model with two related and interdependent computations that you wish to solve self-consistently. These situations would generally be handled in different ways.
Anyway, I'll take a wild guess and assume your models are completely separate and independent of one another (aside from covering identical spatial regions, at least in regard to where the space charge is located), and are thus most likely prepared as two different .mph files. In that case, my approach would be to: (1) simply plot and export to a file, the computed charge density data (as data) that you mentioned from the first model, then (2) import the data from that file into the second model, as part of my definition of an interpolation function, with its "data source" as the file that was exported from the first model, then (3) under space charge density (such as in the electrostatics formulation), specify the charge density to be given by the interpolated function (e.g, int1(x,y) ). I've tested this in 2D and it works just fine. Just make sure the two geometric spaces match and you have a fine enough mesh, etc. Of course, if you are trying to do something totally different, then you may wish to add more explanation here and/or post your model to the forum. Good luck.

I had no idea I could create an interpolation function with given data! Also your guess of the problem (doing the simulation in two separate files) was correct! Thank you for your help.