How to assay the potency of  Dextran 70 content and
Hydroxypropylmethylcellulose content when both components are already
in the solution.

Here is an answer based on knowlegde i have.. :

Since HPMC has more affinity for epithelial cells than Dextran 70, you
can set up a chromatography column with fresh cheek epithelial cells
inside and the result of this chromatography would be that the Dextran
would simply pour out while the HPMC will come out later (since it
will travel slowly in the column).

I really couldnt find any online articles based on this, but this might help

Sahil