Depending on what charger you use & how you balance, it could be a bad thing.

If you are putting out a high rate of charge & the balancer cant keep up, you will likely over charge the heck out of one pack...

Both packs should be fairly close, & if they are not... you can force a discharge separately on each one first to any voltage you like 3.7? 4.0? & then recharge them as a pair.

If your charger can auto discharge for you it is that much easier.

Most chargers don't care what cell is where as far as voltage, it will likely hard charge until near 8.4v & then roll back amperage until a full charge.

I ran all Summer with two 2s as a single 4s... I've had good results.

I like to see the outcome on a 1/2 & 3/4 pair, I'm not willing to try.
The amount of amperage isn't safe if you plug a dead pack into a 3/4 full one.
You could with NiMh or NiCd, they just get hot...
Don't do it with LiPos...

You may be able to do it in the future with LiFe packs... who knows yet...