Short answer is don’t do it.
The longer answer is the higher voltage pack will charge the lower one and probably damage it.
The Long answer is maybe if you put a reverse protection diode in each string so the higher voltage string can not discharge into the lower one and blow it up. But then the diodes will have a voltage drop and need to be sized for current and you will draw 100% of your power from the higher voltage pack until it drops, then you will share for a little bit, then whichever one goes lower first will drop out and finally you will oscillate between the two. You would never get double the power buy paralleling the battery packs.
The complex answer is if each battery pack feeds it’s own voltage regulator and they are setup in a master/slave configuration so they both maintain the same output voltage onto the power buss then it can work. But at this point is probably cheaper to get two battery packs with the same chemistry batteries.
One other thought, if you are paralleling two battery packs and they are physically separate you should ensure the connection is using low resistance or equal resistance wire otherwise you will get something very similar to having packs of different voltage.
Wires (a) should be the same size and length as wires (b).
Bat 1 ----(a)-----+
------ to powered device
Bat 2 ----(b)-----+