This page gives some insight into the important factors to consider when using batteries in parallel. The upshot is basically that even batteries of the same chemistry and brand can have different voltages if they’re different ages/have been used different amounts. If you measure their voltages and they’re exactly the same then it might be ok, but that also can’t be guaranteed over time because some batteries will lose capacity faster than others, even within the same pack.
This is part of why most Li-Ion battery packs come with a built in battery management system (BMS), to help ensure that the different battery cells are kept balanced and maximise battery performance and longevity. You might be interested in something like @seaview’s Multi-Battery Manager, although looking at their website it’s recently been discontinued in favour of their new/upcoming Power System Manager, which @gcelec might be able to provide some more information about 