I need more current but I do not want to change my pcb board so can I piggy back two L293D pin for pin? I am driving dc motors and I am not seeing any gross failures but I am very close to the max ratting and so I wanted just a bit more headroom.
That's not generally recommended. If you know the devices are from the same batch (i.e. are more closely matched) and you can ensure that they are thermally coupled (hard to do since they are in packages) then it's not recommended.
It would safer to get another similar device that has lower Rds(on).
I've definitely seen this done with exactly this chip, so it can work. As rawbrawb indicates, though, it's a hack - it might work, but it might cause problems due to mismatched components. Your worst case scenario: you burn out both of them from excessive current. Since they're cheap parts, it might be worth a try anyway, depending on your constraints.
If you have through hole chips, it could be feasible to increase the amount of power on your board in a safer way than soldering two chips on top of each others.
As explained in the datasheet, the safe way to double the power of l293 chips is to use 1 chip per motor. To do that, you have to connect channel 1 to channel 4 and channel 2 to channel 3.
In other word, if you want to connect two chips, you'll have to solder them above the board next to each others. One chip for the left side and one chip for the right side.
The tricky part is to have both pin enable, vcc connected correctly as they are not parallel to each others.
I'd flip the chip on the side like this:
The advantage of this method against piggy backing two chips is that if one chip fails, then the second motor will still work. And you only have to replace one chip.
One way that would look less like a hack would be to have a new PCB that you can stack on top of the l293d socket.