Have you considered Go To Related Records in a script?
Do a find to pull up the desired proposals records, then click a button to perform a script that uses Go To Related Records with the "Match All Records in Current Found Set" option to switch to your investors layout and show only the investigator records matching the proposals records you just found.
Go To Related Records is a very useful tool, but which is very poorly documented. To learn more about GTRR, click the following link:
The Complete Go To Related Record