Packs need to be reasonably close in voltage before connecting. Most suggest within 0.1V, some go further.

This is probably the safest way, fused and rated for 40 amps:



Revolectrix Multi-Port Safe Parallel Adapter (MPA)

Progressiverc FMA safe parallel adapter for jst-xh t-plug

More options, not fused and 30 amp rated:



EP Buddy ParaBoards

Progressiverc parallel-charge-boards