(ET) mixing 6 and 12v batteries?

[tb <tbamc gbta net>]

My battery pack is made up of varying vintages and brands.  4 still have 
at least a year or 2 of life (and maybe more) left but the 2 newest (only 
2 years old, and no winter use) are awful.  Whether charged with the pack, 
or individually with a smart charger, they won't get much above 6.2v or 
so.  They aren't USB's but they were new when I bought them--from the same 
guys I generally get USB's from-- and they were close to this bad most of 
last year.  Not sure if I undercharged them the first year I had them or 
if something else is going on?  

If money were no object I'd spend $600 and replace the whole pack, and be 
a happy camper.  But it is.  I'm not keen to spend $200 for 2 new 
batteries which obviously won't match the old, etc. etc.  Have looked 
around for used but none available.   So my question is, as a somewhat 
short term solution would replacing the 2 bad 6v batteries with one 12v 
battery make any sense, or be a terrible idea?  I'm guessing the 12v might 
be cheaper  than 2 sixes?  And then, at the end of the rest of the packs 
life, I'd theoretically have a 12v battery that might have some life in 
it, for other uses?

Thanks.

Thon