I can say that Schopenhauer would feel contempt for a man like Marx. The reason for that can be found in fact that Marx's teaching about collective needs in case of human dialectic directly violates Schopenhauer's concept of freedom and right which both contents are negative, i.e. there must be absence of any hindrance for activity in the case of freedom and of any injury in the case of right.
What Marx meant was some kind of positive freedom and right, that means that people or institutions should intervene in changing the society and its functions to create a classless society.
Other factor for which Schopenhauer would feel disdain is the aim or end of Marx's dialectic, that is the rule that all people shpuld create wealth collectively. As a proponent of limited government which only aim should be to enable man free to work out his salvation Schopenhauer would definitely ranked Marx next to Hegel, as a clumsy charlatan.
There is a passage in his Parerga and Paralipomena where he expresses his negative view on workers' movement with the aim to rule the state. He even said in his masterpiece The World as Will and Representation that he would prefer to be ruled by a lion than one of his fellow rats.
Regarding you question about the fact 'that both felt that these drives must be objectified externally to be actualized', I am of opinion that you did not grasped Schopenhauer's concept of the will right.
The will for him is already objectified in different grades from inorganic nature (like simple matter) to the highest grade in organic nature (man). And the aim for him is not for a people to actualize this will to live (that was a concept Nietzche invented) but to resign from it and attain a blissful state of nirvana, the same aim as in Buddhism.
So they were in direct opposition in basis of their teachings.
No comments on eachother were found by me but I might even add that events in a period of histoy after both men proved who was right.