It is pretty clear to me that Sanders and Warren are working together. I suspect one of them is going to drop out and the one who drops out will endorse the other. Its likely going to be Sanders dropping out and endorsing Warren. It will sort of be like when Chris Christie dropped out and endorsed Trump.
They might end up running mates but, they will not make it out of the gate. Their campaign funds will dry up as the big moneyed extreme liberals will not support them financially! Also, expect attack ads to come out against them if they somehow end up getting some support among liberal voters. Those Democrat Super Pacs are going to target them if they manage to somehow get far in the fight for the Democrat nomination.