As the design has just started, I suggest, why don't you at the same time design to have an extra battery with the qwerty OtherHalf. The extra battery could be the same as the original Jolla battery, doubling the mAh value.

I have noticed, I have never been satisfied with the original battery capacity of any recent mobile devices I've been using. I have purchased a bigger battery to all of these: Nokia 8110, Nokia 6260, N95, N95 8GB, N900, Samsung Galaxy S3. I guess I have been and always be a heavy user, whether it was Symbian, Maemo, Android, ...(Sailfish, Tizen, ...).

Although Jolla has designed the OtherHalf system to be stackable, so able to stack several OtherHalfs together, I find it unlikely and perhaps unpractical to do for real, for example having extra battery OtherHalf and qwerty-OtherHalf from separate suppliers and then use them with Jolla phone.