Even in Ni-Cd or NI-Mh cells in series, the eventuality of some cells discharging faster, and thereafter the load current (though reducedto some extent), chrging these specific cells IN REVERSE, is not uncommon.
Such being the case Paralleling two sets of batteries, whether same capacity or make or otherwise, will have ODD consequences.The OP can, if he still wants, enjoy this experience at some cost. But such cases it is better to learn from scientific logic and at the worsrt from others experiences, rather than spoiling few more cells . these cells will be spoiled, we won't realise the reason and helplessly throw them.