No, not true IME, as per BA Loyal's point. Whenever I've TCP'ed a booking, I've been able to put the other person wherever I want in the cabin.

Hence, for an upcoming trip to LAS, I've put myself in 2K and my friend, who's a blue on a separate PNR, in 2J - saving however much in seat selection thievery.

However, to answer the OP's question, as the others point out it's only Golds and not Silvers who can do this.