[Patrick]

It is recommended to use packs with the same amount of use, because the older pack might not hold voltage as well and be more likely to drop below 3v per cell. Packs loose a bit of capacity with age as well, making that more likely. But as long as there not miles apart it terms of use, you don't try pulling huge amps from them and you not trying to get every milli amp out of it, then you should be ok. I've done it before as well with packs that were a couple of months different in age.