BMW & JLR are in the hole for €hundreds millions come 2021 for missing their fleet emission standards, BMW much more so as they manufacture many more vehicles.
They're merging to reduce costs of developing electric drivetrains & PHEV vehicles.
Most if not all of the big car manufacturers will miss these 2021 emission targets (BMW are at the bottom of the list) & some of them are merging to reduce the costs of transitioning over to BEV's.
For example FCA (Fiat Chrysler Automobiles) have teamed up with Tesla to pool their fleets together to avoid paying emission requirement fines to the tune of £2billion for emission credits.
http://www2.paconsulting.com/rs/526-...ort%202017.pdf