okay you are not selling shares. that doesn't mean it's not possible to pool the funds according to proportional interest. you can set up the account agreement so that each client owns his or her proportional interest. for example, the existing pool has $100K of securities with 1 client. Client 1 owns 100% of the $100K worth of securities. Client 2 comes along and invests another $100K. The pool now has $200K and each client owns 50%. Both Client 1 and Client 2 now own $50K of securities + $50K cash. Both clients will have the same returns going forward. what is wrong with this? why can't the brokers make this happen? it can be easily explained, much easier than explaining why client 2's account is down while client 1's account is up for the month.