Was very interested in this as well when I saw you
Post# of 250601
TDA and E*Trade both confirmed that shares can be transferred from one external brokerage account to another and vice versa.
From what I understand, the OTC Security (penny stock) needs to be accepted at both brokerages and there is no fee. So if both allow a purchase on said security, shares are able to be transferred between accounts. Stocks are moved by requesting the transfer at the receiving firm and both the trading accounts need to be similar. The function is found under partial account transfer at the receiving broker.
Not sure if this fits your scenario precisely, but I learned something new today and will keep this in mind as an option to move some shares around.