Ahh, yes, in Freemind there is, strictly speaking, only one parent. I think they adopted this restriction because folding/collapsing of nodes is hard to define and/or hard to understand without this restriction, and folding is an important part of helping mind naturally grasp the big "picture".
However, within the visual Freemind diagram it is also possible to create arrows from one node to another that are not part of the main restricted hierarchy. I know some users have created these with the intent of creating multiple parents for a single node. So I would say that although Freemind doesn't support it very well (i.e., it's a hack, not natural like with graphviz), it does support showing visual relationships of single child with multiple parents. (And as far as export to Freemind from orgmode goes, I expect the export could be written to include arrows that are intended to show multiple parents of a single node, although I'm pretty sure it doesn't right now.)
However, within the visual Freemind diagram it is also possible to create arrows from one node to another that are not part of the main restricted hierarchy. I know some users have created these with the intent of creating multiple parents for a single node. So I would say that although Freemind doesn't support it very well (i.e., it's a hack, not natural like with graphviz), it does support showing visual relationships of single child with multiple parents. (And as far as export to Freemind from orgmode goes, I expect the export could be written to include arrows that are intended to show multiple parents of a single node, although I'm pretty sure it doesn't right now.)