Lithium Manganese Dioxide Battery (Li/MnO2) - Maxell
Using different batteries together, i.e. different type or used and
new or different manufacturer could cause distortion, leakage,
overheating, explosion, or fire because of the differences in battery
property. If using two or more batteries connected in series or in
parallel even same batteries, please consult with Maxell before
The danger with using cells in parallel is that if one has less charge then the other one will attempt to charge it. This could occur if one battery holder has a poor contact which causes the cell to discharge slower, or if they do not have the same state of charge when installed.
A safer solution would be to use a single higher capacity battery - and if the size of your device has to change to fit it then consider using a different battery type.