I agree with Tom, but think the following would apply and may be where Reno was going.
I would think you could say the rods at the house augment the one at the outbuilding but only if the EGC in the feeder is of sufficient size to qualify as an electrode bonding conductor. For a feeder with #8 ungrounded conductors, you need a #8 EGC in order to count it as an electrode bonding conductor. The #10 the OP used would not be large enough. I think the intent is that the augmenting rod be near the detached structure but no closer than 6' to the primary rod. But there is no code I can find that mandates a maximum limit on the distance.
