3-way thick wire connector (8-10awg wires)
There are "multiway" crimp butt connectors designed to accept two wires at one end and one at the other. For example, these ones. 



Called "In-line, Multi Wire Butt Connectors, Heat Shrink, Crimp Seal", they have heat shrink insulation with adhesive to seal the crimp joint. 

Their largest size will accept 2x AWG 10 to 1x AWG 8 wires.