Wiring lithium packs together?

Not sure if this is the correct forum. Assumed custom build could be for batteries. My question is this, could two separate, already assembled, in their own cases lithium packs be wired together to run in series or parallel? Basically what I want to do is run both power connections from the packs into some kind of "Y" adapter into the single power connection on the controller. This way I could either use them at the same time for more power or individually for more range. My main concern would be any kind of electrical feedback between the batteries which could result in shorting them out. If this is possible, can anyone in here recommend a controller with a wide range from 36 to 72 volts? This is a concern due to some controllers having low voltage cutoff. The batteries will be a silverfish style pack from an LX1 bike (36V 10Ah) and a little frog style pack (39V 8Ah). I found the 39V battery on Dhgate for around $221.00 free shipping if anyone is interested. Just type in 39V 8Ah in their search and it should be the only item that pops up. I was amazed by the low price on the pack so I'm hoping it's not a piece of crap. Any help, "either positive or negative", would be appreciated. Little pun there.