As I said, paralleling batteries works only in a perfect world and relies on near perfect matching. It's a stupid and dangerous design practice to use especially for the inexperienced but like I said: just my opinion.
If you are asserting that it is OK to parallel a six-cell 7.2V battery and a seven-cell 8.4V battery, I think that's sufficient. Go ahead. One cell in the 8.4V has probably been killed (shorted) in the process if they function in parallel, but whatever works for you.
Couple of things ...
The danger aspect you allude to makes no sense. The rapid chargers used for NiMH often make the batts VERY warm -- this never happens with running respective NiMH batts in parallel (they don't even feel mildly warmer in parallel (than single)). Since heat is an enemy of batt. physical chemistry, the chargers themselves pose a
much greater relative "threat" if one is concerned with explosion/leakage/fire hazard as well as cell longevity.
The 9v species of NiMH -- as I noted earlier --
seems to be troublesome to engineer. Many major manufs of NiMH -- Sanyo, Duracell, etc. -- tend to stay clear of offering it in NiMH.
I don't understand your comment " One cell in the 8.4V has probably been killed (shorted) in the process if they function in parallel...". I have quite few brands/models of 9v batts and chargers in my "collection". When one of the 1.2v in-series cells does go bad ("cell reversal", etc.), the internal cell/batt. impedance messes up and the whole batt. either faults out the charger and/or it does not maintain (hold) correct charge (voltage) over time. And yes, those mini 1.2v internal cells DO go bad JUST AS OFTEN in single (intend, i.e., non-parallel) use, depending on manuf. (e.g., Maha is v. bad; see Amazon reviews).
**broken link removed**
I've researched this topic, fairly extensively I guess -- including direct communication with manuf. engr depts, and referenced various datasheets and topical web sites/blogs, and recent-pub'd battery-technology textbooks, as well as engaged in myriad forum discussions (such as THIS!). Bottom line: it's a gray area ... not a whole lot written about or extensively discussed. Maybe, this is all due to market demand: Li-Po seem to be the hot trend. IAC,and FWIW, Maha Energy says it's okay to parallel.