Hi Jack, I am not sure that this is a good idea as they are two different types of batteries and may not be compatible. Especially when you go to recharge them. You mention hooking them up in series, plus of one battery to the negative of the other. This will double your voltage to 7.4 volts but will not give you more capacity only more voltage. For more capacity the batteries would have to be hooked in parallel or plus to plus and minus to minus. There are guys on the site that know a lot more about these types of batteries that I do. I would recommend holding off until one of them looks your situation over.
John