Combining close passage numbers? - (May/03/2015 )

I have an interesting question.  My 184A1 immortalized mammary epithelial cells grow pretty slowly, and I want to do an IP-HAT (immunoprecipitation-histone acetyltransferase) assay.  To do this, I need about 8.0*10^6 cells per treatment condition (that should give me 2000 ug, enough for two 1000 ug pulldowns), so 32M cells would be necessary.  I might have enough to do it with only P40 cells, but just in case, I want to know if I can also throw in my P41 cells (derived from the P40 passage).  I understand that mutations might arise due to passage distance and surface-protein identity may be different due to the extra trypsinization, but I was told by a senior scientist in a nearby lab that the cell identities might be close enough as to be practically indistinguishable (remember, these are immortalized normal-like cells that should have near-normal karyotypes).  Does anyone think there would be any issue with combining the passage numbers and calling the result P42?

As these are immortalized, there shouldn't be any problem with combining the passage numbers, given that the difference in passage should only be a few cell divisions (3-5?)

That's what I thought.  Thanks.