Yes it's doable, BUT it's not advisable.
They'd need to be the same chemistry, ie Lithium, Lead acid etc you can NOT mix types.
you'd need a cell equalizer across the 2 volt portion.
they should be from the same manufacturer and really close to the same age.
Id recommend this ONLY if you already have the batteries and are just looking for how to make it work, Id NOT buy them and do this.
Aaron
thank you. I don't have batteries so I will get the same ones.

