What should we call your proposed coalition - dhimmis anonymous? Leftists do actually have a religion of sorts, though I suppose the correct term would be an anti-religion. Leftists are never going to get it. The underlying reason is that they seek the same goal that the Muslims do - the complete destruction of the west. The evil oppressor/patriarchy/untermenschen must be destroyed in order for the free and equal supermen/oppressed/übermenschen to emerge and establish utopia. Leftists see Muslims as their great allies amongst the "oppressed other" in the master plan.
Actually the atheists should form an alliance with the Muslims, since they'll go after everybody indiscriminately, including themselves. Look at how kind and tolerant they were in the Soviet Union, Spain, China, Cambodia, Vietnam, etc.